ecological effects of solarization...

ECOLOGICAL EFFECTS OF SOLARIZATION DURATION ON WEEDS,

MICROARTHROPODS, NEMATODES, AND SOIL AND PLANT NUTRIENTS

By

RACHEL SEMAN-VARNER

A THESIS PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2005

This manuscript is dedicated to Maya R. Seman-Varner, whose presence in my life has

reinforced the necessity of working for a sustainable future.

ACKNOWLEDGMENTS

This thesis project involved the cooperative effort of many people with diverse

skills. K. Dover, R. Menne, and P. Jackson provided excellent field and laboratory

assistance, even under adverse conditions. K-H Wang assisted in the field and laboratory,

and provided technical and scholarly assistance, as well. J.J. Fredrick provided

meticulous laboratory assistance and was always available to answer technical questions.

J.R. Chichester was essential in all soil and leaf tissue analysis. S. Taylor and the staff at

the University of Florida Plant Science Research and Education Unit in Citra, Florida

provided equipment and expertise in the installation and maintenance of the experimental

site.

I must express my gratitude to the members of my committee, Dr. R.N. Gallaher,

Dr. S.E. Webb, and Dr. R. McSorley, for their advice concerning the design,

implementation, and analysis of this experiment, and for their guidance in preparing this

manuscript. Dr. McSorley deserves special thanks for providing outstanding academic

and scientific advisement throughout my graduate school experience. He and I have

navigated what may have been a non-traditional method of completing a graduate

program with excellent results, in part because of his knowledge, patience, and dedication

as my advisor.

I must also thank my family and friends for their support during graduate school. I

may not have entered this program focused on agricultural ecology if it were not for my

friend and mentor, Dr. R. Koenig. I never would have completed this program without

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the scholarly advice, personal guidance, and loving support of J. Morgan Varner III; our

synergy fuels my work for a sustainable future for our family and the world.

This work was supported by USDA CSREES grant no. 2002-51102-01927.

iv

TABLE OF CONTENTS

Upage

TACKNOWLEDGMENTST ................................................................................................. iii

TLIST OF TABLES T ............................................................................................................ vii

TLIST OF FIGURES T ........................................................................................................... ix

TABSTRACTT.........................................................................................................................x T

CHAPTER

1 INTRODUCTION TO SOIL SOLARIZATION..........................................................1

2 SOLARIZATION EFFECTS ON SOIL FAUNA IN AN AGROECOSYSTEM

UTILIZING AN ORGANIC FERTILIZER SOURCE ................................................6

Introduction...................................................................................................................6

Methods ........................................................................................................................8

Results.........................................................................................................................10

Nematodes ...........................................................................................................10

Total Arthropods .................................................................................................11

Macroarthropods..................................................................................................12

Microarthropods ..................................................................................................12

Collembola ..........................................................................................................13

Mites ....................................................................................................................14

Discussion...................................................................................................................14

Conclusion ..................................................................................................................17

3 WEED POPULATION DYNAMICS AFTER SUMMER SOLARIZATION..........33

Introduction.................................................................................................................33

Methods ......................................................................................................................35

Results.........................................................................................................................37

Weed Cover .........................................................................................................37

Total Weed Populations ......................................................................................38

Individual Weed Species Populations .................................................................39

Weed Biomass .....................................................................................................40

Wind-dispersed Seed ...........................................................................................41

v

Discussion...................................................................................................................41

Conclusion ..................................................................................................................44

4 SOIL NUTRIENT AND PLANT RESPONSES TO SOLARIZATION IN AN

AGROECOSYSTEM UTILIZING AN ORGANIC NUTRIENT SOURCE ............54

Introduction.................................................................................................................54

Methods ......................................................................................................................55

Results.........................................................................................................................59

Soil Mineral Nutrients .........................................................................................59

Okra Leaf Tissue Nutrients .................................................................................60

Okra Biomass ......................................................................................................61

Discussion...................................................................................................................62

Conclusion ..................................................................................................................64

5 EFFECTS OF SOLARIZATION ON RELATIONSHIPS BETWEEN OKRA

BIOMASS, LEAF NUTRIENT CONTENT, AND SOIL PROPERTIES.................72

Introduction.................................................................................................................72

Methods ......................................................................................................................73

Results.........................................................................................................................75

Discussion...................................................................................................................76

Conclusion ..................................................................................................................77

6 CONCLUSIONS ........................................................................................................82

LIST OF REFERENCES...................................................................................................86

BIOGRAPHICAL SKETCH .............................................................................................92

vi

LIST OF TABLES

UTableU UpageU

2-1 Plant-parasitic nematodes during 2003 summer solarization experiment................19

2-2 Plant-parasitic nematodes during 2004 summer solarization experiment................20

2-3 Ring nematodes (Mesocriconema spp.) during 2003 summer solarization

experiment. ...............................................................................................................21

2-4 Ring nematodes (Mesocriconema spp.) during 2004 summer solarization

experiment. ...............................................................................................................22

2-5 Total arthropods (including mites, Collembola, and other Insecta) during 2003

summer solarization experiment. .............................................................................23

2-6 Total arthropods (including mites, Collembola, and other Insecta) during 2004

summer solarization experiment. .............................................................................24

2-7 Macroarthropods during 2003 summer solarization experiment. ............................25

2-8 Macroarthropods during 2004 summer solarization experiment. ............................26

2-9 Microarthropods (Collembola and mites) during 2003 summer solarization

experiment. ...............................................................................................................27

2-10 Microarthropods during 2004 summer solarization experiment. .............................28

2-11 Collembolans during 2003 summer solarization experiment...................................29

2-12 Collembolans during 2004 summer solarization experiment...................................30

2-13 Mites (including non-oribatid and oribatid mites) during 2003 summer

solarization experiment. ...........................................................................................31

2-14 Mites during 2004 summer solarization experiment................................................32

3-1 Horsfall-Barratt ratings for weed coverage during 2003 summer solarization

experiment. ...............................................................................................................45

3-2 Horsfall-Barratt ratings for weed coverage during 2004 summer solarization

experiment. ...............................................................................................................46

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3-3 Weed population (number of plants or stems) during 2003 summer solarization

experiment. ...............................................................................................................47

3-4 Weed population (number of plants or stems) during summer solarization

experiment 2004. ......................................................................................................48

3-5 Number of hairy indigo (Indigofera hirsuta) plants during 2003 summer


3-6 Number of purple nutsedge (Cyperus rotundus) stems during 2003 summer


3-7 Total weed biomass at conclusion of summer solarization experiments during

2003 (56 days post-treatment) and 2004 (67 days post-treatment). .........................51

4-1 Extractable soil mineral nutrient concentrations from 2003 summer solarization

experiment (0 days post-treatment)..........................................................................65

4-2 Soil properties from 2003 summer solarization experiment (0 days post-

treatment). ................................................................................................................66

4-3 Extractable soil mineral nutrient concentrations from 2004 summer solarization

experiment (4 days post-treatment)..........................................................................67

4-4 Soil properties from 2004 summer solarization experiment (4 days post-

treatment). ................................................................................................................68

4-5 Okra leaf tissue nutrient concentrations at conclusion of 2003 summer

solarization experiment (59 days post-treatement). .................................................69

4-6 Okra leaf tissue nutrient concentrations at conclusion of 2004 summer

solarization experiment (65 days post-treatment). ...................................................70

4-7 Dry okra biomass at conclusion of summer solarization experiments, 2003 (56

days post-treatment) and 2004 (67 days post-treatment). ........................................71

5-1 Cowpea green fertilizer nutrient concentration and content for each 3.5 kg mP

-2P

application. ...............................................................................................................79

5-2 Correlation coefficients (r) and levels of significance (p) of the relationship

between okra leaf tissue nutrient content and extractable soil nutrients in

solarized treatments..................................................................................................80

5-3 Correlation coefficients (r) and level of significance (p) for correlations between

leaf tissue nutrients and soil minerals in solarized and non-solarized treatments....81

viii

LIST OF FIGURES

UFigureU UpageU

3-1 Daily maximum soil temperatures at 10 cm during 6-week solarized and non-

solarized treatments in 2003.....................................................................................52

3-2 Daily maximum soil temperatures at 10 cm during 6-week solarized and non-

solarized treatments in 2004.....................................................................................53

ix

Abstract of Thesis Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Master of Science

ECOLOGICAL EFFECTS OF SOLARIZATION DURATION ON WEEDS,

MICROARTHROPODS, NEMATODES, AND SOIL AND PLANT NUTRIENTS

By

Rachel Seman-Varner

December 2005

Chair: R. McSorley

Major Department: Entomology and Nematology

Soil solarization is an alternative to environmentally harmful soil fumigants. This

study was designed to optimize the duration of solarization treatment for the management

of various species and crop nutrients in an agroecosystem utilizing an organic nutrient

source in North Florida. Field studies were conducted during July and August of 2003

and 2004. The experiment was designed as a split-plot with duration as the main effect

and solarization as the subeffect. Five replicates were arranged in a randomized complete

block design on the main effect. Solarization treatments of 2-, 4-, and 6-week durations

began on sequential dates and concluded in mid August both years. Immediately post-

treatment, okra seedlings were transplanted into a 2-m P

2P subplot for tissue nutrient

analysis at final harvest. Freshly chopped cowpea hay was applied to the soil surface

directly around the okra seedlings.

The effects of solarization on several groups of organisms were studied. Nematodes

and soil microarthropods were collected from composited soil samples from a 2-m P

2P

x

subplot. Nematodes were extracted from soil using a modified sieving and centrifugation

technique. Mesofauna were extracted using a modified Berlese funnel. Weed cover,

density, and individual weed density were determined from 1-m P

2 Psubplots. Weed biomass

was measured from 0.25-m P

2P subplots within 1-m P

2 Psubplots at final harvest. Soil nutrients

and properties, and okra leaf tissue nutrients were determined using standard laboratory

methods. Okra biomass was measured at the conclusion of the experiment.

Although the population of nematodes was relatively low at the beginning of the

experiment, solarization delayed nematode recolonization. Collembola and mites were

reduced by solarization and the rate of recolonization was also reduced, up to 2 months

after treatment. Weed cover and density were reduced by solarization, but density of

particular species responded differently to treatment. Weed biomass was reduced by 90%

in solarized treatments at final harvest compared to non-solarized treatments.

Concentrations of soil Mn and K increased with solarization, which may have caused the

increase in leaf tissue Mn and K concentration. Overall, there were stronger relationships

between soil nutrients and leaf tissue nutrients in solarized treatments than in non-

solarized treatments.

Based on data from this experiment, 4- and 6- week durations of solarization were

optimal for managing weed density and increasing crop biomass. Availability of nutrients

from an organic source was not limited by decreases in soil fauna due to solarization,

which may suggest the importance and recovery of fungal and bacterial detritivores. With

continued research, solarization can be optimized to manage a variety of specific

organisms or plant nutrients and applied as an effective, environmentally-sound

alternative to soil fumigants.

xi

CHAPTER 1

INTRODUCTION TO SOIL SOLARIZATION

Environmental concerns have led to recent restrictions on soil fumigants including

methyl bromide. Although fumigants like methyl bromide effectively control soil-borne

pests with low initial costs, the cost of ozone depletion caused by these fumigants is

much higher than the investment in alternative methods (Thomas, 1996). Restrictions on

fumigants and environmental concerns have increased research emphasis on non-

chemical methods of soil disinfestation. Modern soil solarization, a non-chemical

disinfestation technique, developed in the 1970s and expanded to 38 countries by 1990

(Katan and DeVay, 1991). Within the last quarter century, soil solarization has been one

of the most researched non-chemical methods of soil-borne pest management and has

been used in the successful management of fungal and bacterial pathogens (Katan, 1987;

Davis, 1991; Shlevin et al., 2004), nematodes (Stapleton and Heald, 1991; McSorley,

1998), insects (Ghini et al., 1993), and weeds (Standifer et al., 1984; Elmore, 1991;

Patterson 1998).

Soil solarization involves the application of thin (25-30µm), clear polyethylene or

polyvinyl chloride plastic that transmits solar radiation to heat the underlying soil that

subsequently kills soil-borne pests (Katan, 1981; Katan and DeVay, 1991; McGovern and

McSorley, 1997). During solarization treatments, the upper 30 cm of the soil is heated to

temperatures usually within the range of 30 to 60°C, depending on soil type, soil

moisture, climate, and treatment enhancements (Katan, 1981). Over the course of

treatment, generally 4 to 8 weeks, the soil is diurnally heated with temperatures highest at

1

2

shallower depths and peaking in the afternoon. The cyclical heating of the soil results in

lethal temperatures sustained long enough to manage a variety of pests.

Specific methods to increase the effectiveness of solarization have been defined

(Stapleton, 2000). Orienting solarization beds in a north-south direction may increase

effectiveness of solarization by increasing intensity of heating in the bed shoulders

(McGovern et al., 2004). A double layer of solarization plastic has been used to increase

control of certain pest organisms, such as grasses, fungal pathogens such as Rhizoctonia

spp. and Pythium spp., and some root-knot nematodes (Meloidogyne spp.) (Ben-Yephet

et al., 1987; McGovern et al., 2002). Increased soil moisture before plastic application

has increased the conduction of heat and the sensitivity of pest organisms (Katan, 1981).

Amendments including compost, crop residues, and animal manures may also increase

the effects of solarization (Gamiel and Stapleton, 1993; McSorley and McGovern, 2000;

Coelho, 2001). Once the plastic is removed, the soil is disinfested and ready for planting.

Some of the earliest solarization research examined several species of fungal and

bacterial pathogens. Verticillium dahliae Kleb., a fungal pathogen, was controlled by

solarization even at lower temperatures (< 37°C) and greater soil depths (up to 70 to 120

cm; Pullman et al., 1981; Ashworth and Gaona, 1982; Davis and Sorensen, 1986).

Common scab, (Streptomyces scabies (Thaxter) Lambert and Loria)) a bacterial

pathogen, was also suppressed by solarization (Davis and Sorensen, 1986). Extensive

empirical evidence found solarization effectively controlled many other pathogens

including Fusarium spp., Sclerotium spp., Phytophthora cinnamomi Rands, Pythium

ultimum Trow, Agrobacterium tumefaciens (Smith and Townsend) Conn (Katan, 1987;

Davis, 1991).

3

Phytoparasitic nematodes are among the most studied organisms in solarization

research. The management of many nematode genera by solarization has been successful,

while some genera have proven more difficult to manage (Stapleton and Heald, 1991).

Long-term control of nematodes by solarization has proven inconclusive, while short-

term control has been successful in increasing crop yields, even under sub-optimal

conditions (Overman, 1985). Several studies have incorporated additional management

techniques with solarization that have increased the success of nematode management

such as cover cropping, planting resistant cultivars, and using a double-layer of

solarization plastic (Chellemi et al., 1993; McSorley, 1998; McSorley et al., 1999;

McGovern et al., 2002).

Soil-borne arthropods have not been a focus of solarization research, but they

warrant study because of their role in nutrient cycling and in the regulation of populations

of other soil organisms. Ghini et al. (1993) found that solarization reduced

microarthropods in plots solarized for 30 and 50 days. Studies on lethal temperatures in

insects and mites have found that species are generally unable to complete their lifecycle

if temperatures are elevated between 35 and 55ºC (Fields, 1992). A delicate balance

exists between populations of soil microarthropods, which include nutrient-cyclers,

phytoparasites, predators, and fungivores. Information on solarization and temperature

effects on individual species can be applied when considering solarization effects on the

ecology of the soil.

Weeds are often a major pest problem in areas where solarization is applicable, due

to the optimal climate for both. Several studies have found suppressive effects of

solarization on a variety of weed species including winter annuals (e.g. Senecio spp.,

4

Lamium amplexicaule L., Poa annua L.), summer annuals (e.g. Cyperus spp., Eleusine

indica L. (Gaertn.), Amaranthus spp., Chenopodium album L.), and perennials (e.g.

Cynodon dactylon (L.) Pers., Convolvulus spp., Sorghum halepense L. (Pers.); Elmore,

1991). Several mechanisms of weed control by solarization have been proposed. These

mechanisms include direct thermal damage to seeds, germinating seeds, and shoots;

morphological changes in shoots and other plant organs; breaking of dormancy and

germination at greater depths; an imbalance of gases in soil; and indirect effects on soil

microorganisms that might be seed pathogens (Rubin and Benjamin, 1984; Elmore, 1991;

Chase et al., 1998).

Research has proven solarization to be an effective method for the management of

a variety of pest species. Because solarization requires relatively clear skies, high

temperatures, and a 4 to 8 week treatment period to be most effective, there are

limitations on the regions where the method can be applied. Recent work has focused on

the development of recyclable, reusable, or biodegradable plastics, which would

minimize waste products of solarization (Stevens et al., 1991). Even with these

limitations, solarization remains an effective and environmentally sound alternative to

soil fumigants.

Solarization is of particular interest as an alternative to methyl bromide in organic

agricultural production. However, many organic production systems utilize nutrients

from organic sources such as green manures, plant residues, and compost, which may

cause concern about the effects of solarization on nutrient cycling in this type of system.

Nutrient release from organic sources requires decomposition of the material, primarily

by bacteria and fungi, and subsequent mineralization for nutrient uptake by plants

5

(Powers and McSorley, 2000). Furthermore, the soil fauna is essential to accelerate the

decomposition of organic matter. Mesofauna, which includes nematodes, collembolans,

mites, and others, balance nutrients in the soil either directly through the decomposition

of organic matter or indirectly by regulating populations of fungi, bacteria or other soil

fauna (Coleman and Crossley, 1996; Larink, 1997). If solarization functions to sterilize

the soil and damage populations of organisms responsible for decomposition and nutrient

cycling, it is possible that the release of nutrients from an organic source could slow or

stop.

This project was designed to answer several questions regarding the use and effects

of solarization. The main objective was to examine the ecological effects of three

durations of solarization treatment on populations of weeds, populations of soil fauna,

soil chemistry, and crop nutrition. Additionally, the project was designed to test

solarization effects on individual groups of arthropods, nematodes, and weeds, with the

intention of recommending an optimal duration of treatment for specific target organisms.

Finally, the experiment examined the effect of solarization on the availability of nutrients

from an organic fertilizer source, for application in organic production systems.

CHAPTER 2

SOLARIZATION EFFECTS ON SOIL FAUNA IN AN AGROECOSYSTEM

UTILIZING AN ORGANIC FERTILIZER SOURCE

Introduction

Soil solarization has been a successful, well-studied technique for the control of

soil-borne pests (Katan, 1981; Davis, 1991; Elmore, 1991; Stapleton and Heald, 1991;

McGovern and McSorley, 1997). Solarization has been shown to successfully manage

bacterial and fungal pathogens (Hartz et al., 1993; Shlevin et al., 2004), weeds (Standifer

et al., 1984; Patterson 1998), and nematodes (Chellemi et al., 1993; McSorley et al.,

1999; McGovern et al., 2002). Some studies have defined specific methods that increase

the effectiveness of solarization (Stapleton, 2000), including bed orientation (McGovern

et al., 2004), solarization plastic structure (Ben-Yephet et al., 1987; McGovern et al.,

2002), soil moisture, and amendments (Gamiel and Stapleton, 1993; McSorley and

McGovern, 2000; Coelho, 2001).

Phytoparasitic nematodes are among the most studied organisms in solarization

research. The management of many nematode genera by solarization has been successful,

while some genera have proven more difficult to manage (Stapleton and Heald, 1991).

Some ectoparasitic nematodes have been shown to move to greater soil depths but still

were effectively controlled by solarization (Stapleton and DeVay, 1983; Stapleton and

Heald, 1991). Long-term control of nematodes by solarization has proven inconclusive,

while short-term control has been successful in increasing crop yields, even under sub-

optimal conditions (Overman, 1985). Several studies have incorporated solarization with

6

7

other management techniques that have increased the success of nematode management

such as cover cropping, resistant cultivars, and double-layer solarization (Chellemi et al.,

1993; McSorley et al., 1999; McGovern et al., 2002).

Arthropods are not often a focus of solarization research, but warrant inclusion

because of their role in nutrient cycling and in the regulation of populations of other soil

organisms. Ghini and others (1993) found that solarization reduced microarthropods in

plots solarized for 30 and 50 days. Bruchids (Callosobruchus maculates and C.

subinnotatus) exposed to 45 and 50ºC for 2 to 6 hours laid fewer eggs and fewer adults

emerged than those exposed to 40ºC (Lale and Vidal, 2003). Another study on lethal

temperatures in insects and mites of stored products found that species are generally

unable to complete their lifecycle if temperatures are elevated between 35 and 55ºC

(Fields, 1992). Oribatid and tetranychid mites show a change in respiration and

reproduction above 29ºC and are killed at temperatures above 35ºC (Stamou et al., 1995;

Bounfour and Tanigoshi, 2001). Information on solarization and other heat effects on pest

species can be applied when considering solarization effects on beneficial species.

Although few studies have included the effects of solarization on beneficial

organisms, beneficials play a fundamental role in managing soil pest populations and in

nutrient cycling. Important arthropod groups including Collembola, oribatid mites, and

non-oribatid mites (Astigmata, Mesostigmata and Prostigmata) are directly involved in

the breakdown of plant residue or indirectly involved by influencing populations of fungi,

bacteria, and other organisms that cycle nutrients directly (Larink, 1997; Maraun et al.,

1998). The effects of solarization on beneficials are important to consider under any

8

circumstance, but especially when working in a system that uses an organic fertilizer

source.

This study was designed to add to our understanding of solarization effects on soil

fauna. The main objectives were to measure the effects of solarization at varying

durations on pest organisms (nematodes and macro-arthropod pests) and beneficial

organisms (especially Collembola, oribatid mites and non-oribatid mites). Our specific

hypotheses were that solarization would decrease plant-parasitic nematodes, pest insects,

and beneficial soil fauna, and therefore affect the availability of an organic fertilizer.

Further, we hypothesized that the greater the duration of solarization, the greater the

delay in recolonization by soil fauna.

Methods

The experiment was conducted at the University of Florida Plant Science Research

and Education Unit near Citra, Florida during the summers of 2003 and 2004. The soil

type was a hyperthermic, uncoated, typic Quartzipsamments of the Candler series with a

0 to 5% slope (Thomas et al., 1979). Average soil pH measured at the study site was 5.9.

The measured soil texture was 95% sand, with 3% clay and 2% silt. The field was

prepared with a crimson clover (Trifolium incarnatum L. ‘Dixie’) cover crop during the

winter season and disked two days before the first plots were constructed.

The experiment was conducted in a split-plot design with duration of treatment as

the main effect and solarization as the sub effect. Five replicates were arranged in a

randomized complete block on the main effect. Each experimental plot was a raised bed 6

m long, 1 m wide, and 20 cm high. The soil was moistened by overhead irrigation if it

was not sufficiently moist before the application of solarization plastic. The solarization

plastic was a single layer of clear, 25 µm-thick, UV-stabilized, low-density polyethylene

9

mulch (ISO Poly Films, Inc., Gray Court, SC). Solarization treatments and non-

solarization control treatments of 2, 4, and 6 week durations were applied during July and

August of 2003 and 2004. The treatments began on sequential dates and concluded in

mid-August both years.

Following solarization treatment, okra (Hibiscus esculentus L. ‘Clemson

Spineless’) seedlings were transplanted into the experimental plots as a bioassay. Okra

was used because it is well-known to be susceptible to root-knot nematodes

(Meloidogyne spp.) (McSorley and Pohronezny, 1981). An organic fertilizer of chopped

green cowpea (Vigna unguiculata (L.) Walp. ‘Iron Clay’) hay, freshly cut at the early

bloom stage, was applied on the soil surface to the area immediately around the okra

seedlings at a rate of 3.5 kg m P

-2P.

Six soil cores 2.5 cm in diameter and 15 cm deep were collected from the

rhizosphere of the okra plants in each plot. The soil cores were composited and samples

were removed for arthropod and nematode extraction. Samples for nematode extraction

were collected on 2 to 3 dates from 0 to 65 days post-treatment. Nematodes were

extracted from a 100 cmP

3P subsample using a modified sieving and centrifugation method

(Jenkins, 1964). The samples were then examined, and nematode genera identified and

counted. Samples for arthropod extraction were collected on 3 to 4 dates from 0 to 65

days post-treatment. Arthropod extraction was accomplished using a modified Berlese-

Tullgren funnel method (Edwards, 1991; McSorley and Walter, 1991). A subsample of

200 cm P

3P of soil was removed and placed on a fine mesh screen in a Berlese-Tullgren

funnel. The soil samples were exposed to a 60-watt light bulb on the funnel for 24 to 36

hours, and the extracted arthropods were collected in a 70% ethanol solution. The

10

samples were then analyzed for total arthropods, Collembola, oribatid mites (suborder

Oribatei), non-oribatid mites (which included suborders Prostigmata, Mesostigmata, and

Astigmata), microarthropods (which included all mites and Collembola) and

macroarthropods (which included all insect larvae, adults, and occasionally hatching

insect eggs and spiders).

During the year of the second field trial (2004) there were several factors that may

have altered conditions at the field site. Three hurricanes in summer 2004 increased the

total rainfall during the experiment [94 cm (37 in)] to more than twice that of 2003 [38

cm (15 in)] (Florida Automated Weather Network, 2005). This may have affected the soil

fauna populations. There was also an accidental application of herbicide to some of the

plots. This was immediately recognized as an error and the area was rinsed with water.

Soil fauna data were compared among durations and between solarized and non-

solarized treatments using analysis of variance. If significant differences were detected

among duration treatments, means were separated using a Least Significant Difference

test at the α = 0.05 level. All data were analyzed using MSTAT-C software (Michigan

State University, East Lansing, MI).

Results

Nematodes

Nematodes recovered included ring (Mesocriconema spp.), stubby-root

(Paratrichodorus spp.), lesion (Pratylenchus spp.), and root-knot (Meloidogyne spp). On

the last sampling date (43 days post-treatment) in 2003, there was a significant reduction

of nematodes due to solarization (p < 0.05; Table 2-1). Ring nematodes were the

dominant genus in 2003 (Table 2-3). In 2004, ring and stubby-root were the dominant

genera, while lesion and root-knot were also present by the conclusion of the study. By

11

the final sampling date (65 days post-treatment) in 2004, solarization reduced total

nematode numbers by 75% and ring nematodes by almost 90% (p < 0.05; Tables 2-2 and

2-4). At the conclusion of the experiment, there were no significant effects of solarization

treatment and no consistent duration effects on stubby-root, root-knot, or lesion

nematodes. The average population numbers of lesion, stubby-root, and root-knot

nematodes were 1.8, 4.0, and 10.9 per 100 cm P

3 Psoil, respectively.

Total Arthropods

During the first field trail (2003), solarization significantly reduced total arthropod

counts immediately post-treatment (0 days) in 2-week and 6-week durations (p < 0.10;

Table 2-5). By 21 days post-treatment, solarization continued to suppress arthropod

populations (p < 0.1). The final arthropod population (43 days post-treatment) showed a

significant interaction; the 2-week solarized plots had significantly lower arthropod

counts than the non-solarized of the same duration and the 2-week non-solarized plots

had significantly higher population counts than either the 4- or 6-week duration non-

solarized plots (p < 0.05).

During the second field season (2004), solarization significantly reduced

arthropod populations immediately following treatment (p < 0.01; 5 days post-treatment;

Table 2-6). By 23 days post-treatment, a significant interaction developed, with no

difference between solarized and non-solarized treatments at the 2-week duration. The 4-

week solarization treatment reduced total arthropod populations 10-fold compared to the

4-week non-solarized treatment (p < 0.05). The 6-week solarization treatment reduced the

total arthropod count by more than 90% (p < 0.1). Of the non-solarized treatments, the 6-

week duration resulted in significantly higher arthropod counts than either the 2- or 4-

week treatments (p < 0.05). At 43 days post-treatment, there were no differences in

12

solarization treatments or among durations of treatment in 2004. At the final sampling

date (65 days post-treatment), there was a reduction in total arthropod population in

solarized treatments compared to non-solarized treatments (p < 0.05). There was also an

increase in the total arthropod counts as duration of treatment increased (p < 0.1).

Macroarthropods

Macroarthropods extracted from soil samples included wireworms (hatching and

larval Elateridae), adult Coleoptera (Scarabidae, Staphylinidae, Carabidae, Elateridae),

Diptera, Hymenoptera, Hemiptera, Dermaptera, and spiders. Although macroarthropod

population levels were relatively low at all sampling dates, there was a significant

reduction in solarized plots compared to non-solarized plots at 21 and 43 days post-

treatment (p < 0.05) during 2003 (Table 2-7). In 2004, macroarthropods were

significantly lower in solarized plots following treatment (5 days post-treatment, p < 0.1),

while at 43 days post-treatment, solarized plots had significantly more macroarthropods

than non-solarized treatments (p < 0.1; Table 2-8).

Microarthropods

At 0 and 21 days post treatment during the first field season, microarthropods were

significantly reduced in solarized treatments (p < 0.1; Table 2-9). At 43 days post-

treatment, the 2-week treatment duration significantly lowered microarthropod

populations in solarized plots compared to non-solarized (p < 0.05).

In 2004, microarthropods were reduced by 93% in solarization treatments at 5

days post-treatment (Table 2-10). At 23 days post-treatment, microarthropod counts were

significantly lower in 4- and 6-week solarized treatments than in non-solarized treatments

of the same durations (p < 0.1). Microarthropod populations in the 6-week non-solarized

treatment were more than 5 times higher than those in the 4-week and nearly 16 times

13

higher than those in the 2-week non-solarized treatment, at 23 days post-treatment (p <

0.05). At 65 days post-treatment, microarthropod counts in solarized plots were nearly

half of those in non-solarized plots. The 6-week duration had the greatest micro-

arthropod population level, while the number of microarthropods in the 2-week treatment

was less than half that of the 6-week treatment and the lowest of the three durations (p <

0.05).

Collembola

Collembola populations were virtually eliminated by solarization in all treatment

durations during the first field trial (2003; Table 2-11). Immediately after treatment, there

was a significant difference between Collembola populations in the solarized and non-

solarized plots in the 2-week treatment, with the 2-week non-solarized treatment resulting

in significantly higher Collembola counts than the 4- or 6-week non-solarized plots (p <

0.05). At 21 days post-treatment, the Collembola populations were significantly reduced

in solarized treatments (p < 0.1). By the final sampling date (43 days post-treatment)

solarization reduced Collembola populations by 76% (p < 0.01). Numbers in the 2-week

treatments were significantly higher than those in both the 4- and the 6-week treatments

(p < 0.1).

In 2004, Collembola counts were reduced by 94% in the 6-week solarized

treatment when compared to the 6-week non-solarized at 23 days post-treatment (p < 0.1;

Table 2-12). The 6-week non-solarized treatment had higher numbers of Collembola than

either the 2 or the 4-week non-solarized treatments (p < 0.05). There were no differences

in solarization or duration effects for the remaining two sampling dates in 2004.

14

Mites

During the 2003 experiment, mites (total consisting primarily of non-Oribatid

mites) had a significantly higher population level in the 2-week non-solarized treatment

when compared to both the 2-week solarized and the 4- and 6-week non-solarized

treatments (p < 0.05), at 43 days post-treatment (Table 2-13). In the second field trial

(2004), mites decreased in numbers in solarized plots at 5 and 21 days post-treatment (p <

0.1; Table 2-14). By the final collection date (65 days post-treatment), mites were

reduced by 55% in solarized treatments compared to non-solarized treatments and by

50% or more in the 2- and 4-week treatments compared to 6-week treatments (p < 0.1).

Oribatid mites were not found in sufficient numbers in either year to permit separate

tabulation or analysis of variance.

Discussion

During both field trials, nematode populations were not immediately affected by

solarization treatments, which may have been due to the relatively low density of

nematode populations in the field site. Pre-treatment nematode counts indicated that ring

nematodes (Mesocriconema spp.) were present, but in relatively low numbers (average of

23 nematodes 100 cm P

-3P of soil in 2003 and 7 nematodes 100 cmP

-3P in 2004), with stubby-

root (Paratrichodorus spp.), lesion (Pratylenchus spp.), root-knot (Meloidogyne spp.),

dagger (Xiphinema spp.), and spiral (Helicotylenchus spp.) also appearing occasionally

(Seman-Varner, unpublished data). It is also possible that the methods used failed to

distinguish between live nematodes and those recently killed, in the samples collected

immediately after solarization (McSorley and Parrado, 1984). However, by the final

sampling date, there were significantly fewer nematodes in solarized plots than in non-

solarized plots, demonstrating that the non-solarized plots were recolonized at a faster

15

rate. The differences in recolonization rates may be due to the higher density of weed

species in non-solarized plots (Seman-Varner, Chapter 3). Although the okra plants were

compromised because of weed competition in non-solarized plots, nematodes had a

higher density and variety of alternative hosts in those plots. Some of the weed species

present were particularly good hosts for root-knot galling and egg development including

pigweed (Amaranthus sp.), purslane (Portulaca oleracea), and clover (Trifolium sp.;

Noling and Gilreath, 2003). Bermudagrass (Cynodon dactylon), one of the most abundant

weed species at the site, is a host for ring nematode in Georgia, USA (Powell, 2001).

These data suggest that solarization is an effective treatment for nematode management

not only because of the direct affect of lethal temperatures on the organism, but also

because of the indirect effects on the soil ecosystem and the interactions between plant

hosts, alternative hosts, and phytoparasitic nematodes.

Initially following treatment, total arthropods were reduced significantly in

solarized plots during 2003. In the final sampling of the 2-week treatment in 2003,

arthropods were more than three times higher in non-solarized plots than in solarized and

almost twice that of any other treatment. Collembola and non-oribatid mites comprised

the majority of the total arthropods in the 2-week non-solarized plots. The cause of the

increase in populations of Collembola and non-oribatid mites is unclear; the opposite

pattern was found in 2004 where the 6-week non-solarized treatments had the highest

arthropod density.

Following the initial reduction of total arthropods in solarized treatments

immediately post-treatment (5 days) in 2004, arthropod populations in the solarized plots

of the 4- and the 6-week durations were also reduced with a greater difference between

16

solarized and non-solarized treatments (23 days post-treatment). The third sampling date

occurred within three weeks of two hurricanes, and there were no significant differences

among any of the treatments. The dramatic increase in precipitation during the

experimental period (more than twice that of 2003; Florida Automated Weather Network,

2005) would have influenced both the establishing populations and further recolonization

of the site by arthropods. However at 65 days post-treatment, which was 2 weeks after

the final hurricane, we saw a significant recovery of arthropod populations in the non-

solarized treatments. Duration of treatment also significantly influenced recolonization,

with the highest population in the 6-week treatment and the lowest in the 2-week

treatment.

Although macroarthropods were found in lower numbers in solarized treatments,

many of them were highly-mobile. The macroarthropod species extracted from the soil

included larvae of Elateridae and Hemiptera that could be pests to many crop species, but

no signs of plant disturbance from insect pests were observed. Further study is required to

make any conclusion about the influences of solarization on macroarthropods.

Microarthropods followed trends similar to those of Collembola because Collembola

comprised the majority of this category.

Collembola populations were reduced by solarization treatments up to 75%. By the

final sampling in 2003, Collembola also showed a significant reduction in the 4-week and

the 6-week durations. The 2-week non-solarized plots had the highest number of

Collembola, possibly because the soil was rototilled immediately before the solarization

plastic was applied to plots, thus incorporating plant residue into the soil providing fungi,

and consequently Collembola, with a food source.

17

In 2004, the second sampling revealed the highest Collembola population in the 6-

week non-solarized treatment. This is the opposite of our findings in 2003 and may have

been influenced by a short-term input of organic matter created by the application of

herbicide. There were no significant differences in Collembola populations between

solarized and non-solarized plots after the second sampling date (23 days post-treatment).

As with the arthropod populations, the increase in precipitation due to seasonal

hurricanes could have influenced establishing populations and recolonizing Collembola.

In contrast with total arthropods, Collembola populations did not recover from the

disturbance to recolonize by the final sampling date (65 days post-treatment).

Mites were consistently reduced by solarization treatments, significantly so on

every sampling date in 2004. Oribatid mites were present in very low numbers and they

did not affect the soil ecology after solarization treatment. Because non-oribatid mites are

primarily fungivores or predators of other mites and nematodes (Coleman and Crossley,

1996), they are likely susceptible to direct and indirect effects of solarization treatment.

Conclusion

These data show the complexity of interactions among soil fauna. Phytoparasitic

nematodes were reduced by solarization, directly through increased temperatures and

indirectly through the management of weed hosts. Collembola are primarily fungivores

that were reduced by solarization. Many non-oribatid mites are predators and followed

similar patterns to the populations of Collembola. Unfortunately, oribatid mites, the

major detritivores in this study were not numerous enough to have much impact on the

soil ecology. Microarthropods recolonized the solarized areas more slowly than the non-

solarized, which suggests a lasting effect of solarization on these organisms more than 2

months after treatment. Despite the reduction of beneficial microarthropods by

18

solarization, nutrient cycling of the green cowpea fertilizer was not reduced, as was

evident from the performance and nutrient analysis of okra plant tissue (Seman-Varner,

Chapter 4). An important element for future study is solarization effects on fungi and

bacteria. These microorganisms may also play a fundamental role in balancing a system

after solarization treatment.

Table 2-1. Plant-parasitic nematodes during 2003 summer solarization experiment.

Nematodes per 100 cm P

3P soilP

z

____________________________________________________________________________________

0 days post-treatment 43 days post-treatment

_____________________________ ________________________________

DurationP

yP Sol Non-Sol Mean Sol Non-Sol Mean

2 9.8 15.2 12.5 13.2 14.2 13.7

4 8.2 9.8 9.0 6.0 17.0 11.5

6 5.0 8.8 6.9 6.4 13.6 10.0

Mean 7.7 11.3 8.5 14.9 *

P

yPDuration of solarized (Sol) and non-solarized (Non-Sol) treatments in weeks, ending on 12 August 2003; post-treatment times

measured after this date. P

z P * indicates significant difference between solarized and non-solarized at p < 0.05. No symbol indicates no significant difference.

19

Table 2-2. Plant-parasitic nematodes during 2004 summer solarization experiment.

Nematodes per 100 cm P

3P soilP

z

__________________________________________________________________________________________

5 days post-treatment 43 days post-treatment 65 days post-treatment

_______________________ _________________________ ___________________________

DurationP

yP Sol Non-Sol Mean Sol Non-Sol Mean Sol Non-Sol Mean

2 8.0 7.0 7.5 4.0 2.2 3.1 17.8 15.6 16.7

4 4.4 2.2 3.3 2.8 9.4 6.1 6.2 71.2 38.7

6 4.4 4.4 4.4 5.8 13.0 9.4 14.2 67.2 40.7

Mean 5.6 4.5 4.2 8.2 12.7 51.3 *

P



zP * indicates significant differences between solarized and non-solarized at p < 0.05. No symbol indicates no significant difference.

20

Table 2-3. Ring nematodes (Mesocriconema spp.) during 2003 summer solarization experiment.

Ring nematodes per 100 cmP

3P soil P

z

____________________________________________________________________________________

0 days post-treatment 43 days post-treatment

_____________________________ ______________________________

DurationP

yP Sol Non-Sol Mean Sol Non-Sol Mean

2 9.8 15.2 12.5 13.2 14.2 13.7

4 8.2 9.8 9.0 6.0 17.0 11.5

6 5.0 8.8 6.9 6.4 13.6 10.0

Mean 7.7 11.3 8.5 14.9 *

P



z P * indicates significant difference between solarized and non-solarized at p < 0.05. No symbol indicates no significant difference.

21

Table 2-4. Ring nematodes (Mesocriconema spp.) during 2004 summer solarization experiment.

Ring nematodes per 100cm P

3P soil P

__________________________________________________________________________________________


_____________________ ______________________ ______________________

DurationP

xP Sol Non-Sol Mean Sol Non-Sol Mean Sol Non-Sol Mean

2 4.0 3.8 3.9 1.2 0.4 0.8 BP

yP 3.0 2.2 2.6

4 3.0 0.6 1.8 1.8 0.8 1.3 AB 2.6 61.4 32.0

6 3.6 3.4 3.5 2.2 5.2 3.7 A 3.6 19.4 11.5

Mean 3.5 2.6 1.7 2.1 3.1 27.7+P

zP

P

xPDuration of solarized (Sol) and non-solarized (Non-Sol) treatments in weeks, ending on 11 August 2004; post-treatment times


yP Means in columns followed by the same letter do not differ at p < 0.05 according to LSD test.

P

zP + indicates significant differences between solarized and non-solarized at p < 0.1. No symbol indicates no significant difference. 2

2

Table 2-5. Total arthropods (including mites, Collembola, and other Insecta) during 2003 summer solarization experiment.

____________________________________________________________________________________________________________

Arthropods per 200 cm P

3P soilP

__________________________________________________________________________________________


_______________________ ______________________ _________________________

DurationP


2 1.6 A 4.2 A+P

yzP 2.9 6.4 6.6 6.5 4.4 A 13.8 A* 9.1

4 3.2 A 2.6 A 2.9 7.4 14.8 11.1 3.0 A 4.0 B 3.5

6 1.4 A 5.6 A* 3.5 5.4 10.6 8.0 3.8 A 7.4 B 5.6

Mean 2.1 4.1 6.4 10.7* 3.7 8.4

P

x PDuration of solarized (Sol) and non-solarized (Non-Sol) treatments in weeks, ending on 12 August 2003; post-treatment times


y PMeans in columns followed by the same letter do not differ at p < 0.05 according to LSD test.

P

zP +, * indicate significant differences between solarized and non-solarized at p < 0.1, and 0.05, respectively. No symbol indicates no

significant difference.

23

Table 2-6. Total arthropods (including mites, Collembola, and other Insecta) during 2004 summer solarization experiment.

Arthropods per 200 cm P

3P soilP

________________________________________________________________________________________________

5 days post-treatment 23 days post-treatment 43 days post-treatment 65 days post-treatment

______________________ ______________________ ______________________ _____________________

DurationP

xP Sol Non-Sol Mean Sol Non-Sol Mean Sol Non-Sol Mean Sol Non-Sol Mean

2 0 3.8 1.9 1.2 AP

yP 1.6 B 1.4 4.0 4.6 4.3 8.2 9.4 8.8 B

4 0.4 3.2 1.8 0.4 A 4.0 B* 2.2 7.4 2.4 4.9 9.6 16.2 12.9 AB

6 0.4 2.2 1.3 1.8 A 20.8 A+ 11.3 2.8 5.0 3.9 11.4 23.0 17.2 A

Mean 0.3 3.1**P

zP 1.1 8.8 4.7 4.0 9.7 16.2*

P



y PMeans in columns at 23 days post-treatment followed by the same letter do not differ at p < 0.05 according to LSD test, means in

columns at 65 days post-treatment followed by the same letter do not differ at p < 0.1 according to LSD test. 24

P

zP +, *, ** indicate significant differences between solarized and non-solarized at p < 0.10, 0.05, and 0.01, respectively. No symbol

indicates no significant difference.

Table 2-7. Macroarthropods during 2003 summer solarization experiment.

____________________________________________________________________________________________________________

Macroarthropods per 200 cmP

3P soil P

__________________________________________________________________________________________


______________________ ______________________ _______________________

DurationP

yP Sol Non-Sol Mean Sol Non-Sol Mean Sol Non-Sol Mean

2 0.2 2.2 0.7 1.6 2.0 1.8 0.2 1.0 0.6

4 1.4 0.4 0.9 0.2 4.0 2.1 0.0 1.4 0.7

6 0.0 0.8 0.4 1.8 3.6 2.7 0.2 1.2 0.7

Mean 0.5 0.8 1.2 3.2*P

zP 0.1 1.2*

P

y PDuration of solarized (Sol) and non-solarized (Non-Sol) treatments in weeks, ending on 12 August 2003; post-treatment times


zP * indicates significant differences between solarized and non-solarized at p < 0.05. No symbol indicates no significant difference.

25

Table 2-8. Macroarthropods during 2004 summer solarization experiment.

Macroarthropods per 200 cmP

3P soil

________________________________________________________________________________________________


______________________ ______________________ ______________________ ________________________

DurationP

xP Sol Non-sol Mean Sol Non-sol Mean Sol Non-sol Mean Sol Non-sol Mean

2 0.0 0.4 0.2 0.4 0.4 0.4 1.0 0.4 0.7 0.4 0.2 0.3 BP

y

4 0.2 0.4 0.3 0.4 0.6 0.5 4.6 0.4 2.5 1.0 1.6 1.3 A

6 0.0 0.2 0.1 0.6 2.0 1.3 1.0 0.6 0.8 1.0 0.8 0.9 AB

Mean 0.1 0.3+P

zP 0.5 1.0 2.2 0.5+ 0.8 0.9

P

x PDuration of solarized (Sol) and non-solarized (Non-sol) treatments in weeks, ending on 11 August 2004; post-treatment times



P

zP + indicates significant differences between solarized and non-solarized at p < 0.10. No symbol indicates no significant differences. 2

6

Table 2-9. Microarthropods (Collembola and mites) during 2003 summer solarization experiment.

____________________________________________________________________________________________________________

Microarthropods per 200 cm P

3P soil

__________________________________________________________________________________________


________________________ _________________________ ______________________________

DurationP

xP Sol Non-Sol Mean Sol Non-Sol Mean Sol Non-sol Mean

2 1.4 3.0 2.2 4.8 4.6 4.7 4.2 AP

yP 12.8 A* 8.5

4 1.8 2.2 2.0 7.2 10.8 9.0 3.0 A 2.6 B 2.8

6 1.4 4.8 3.1 3.6 7.0 5.3 3.6 A 6.2 B 5.0

Mean 1.5 3.3+P

zP 5.2 7.5+ 3.7 7.2

P




P

zP +, * indicate significant differences between solarized and non-solarized at p < 0.10, and 0.05, respectively. No symbol indicates no

significant difference.

27

Table 2-10. Microarthropods during 2004 summer solarization experiment.

Microarthropods per 200 cm P

3P soil

________________________________________________________________________________________________


______________________ ______________________ ______________________ _____________________

DurationP


2 0.0 3.4 1.7 0.8 AP

yP 1.2 B 1.0 3.0 4.2 3.6 7.8 9.2 8.5 B

4 0.2 2.8 1.5 0.0 A 3.4 B * 1.7 2.8 2.0 2.4 8.6 14.6 11.6 AB

6 0.4 2.0 1.2 1.2 A 18.8 A+ 10.0 1.8 4.4 3.1 10.4 22.2 16.3 A

Mean 0.2 2.7*P

zP 0.7 7.8 2.5 3.5 8.9 15.3*

P



y PMeans in columns followed by the same letter do not differ at p < 0.05 according to LSD test at 23 days post-treatment. Means in

columns followed by the same letter do not differ at p < 0.1 according to LSD test at 65 days post-treatment. 28

P

zP *, + indicate significant differences between solarized and non-solarized at p < 0.05, and 0.10, respectively. No symbol indicates no

significant differences.

Table 2-11. Collembolans during 2003 summer solarization experiment.

Collembola per 200 cm P

3P soil

__________________________________________________________________________________________


________________________ _________________________ _________________________

DurationP


2 0.2 AP

yP 2.2 A*P

zP 1.2 0.4 0.4 0.4 1.2 6.0 3.6 A

4 0.0 A 0.0 B 0.0 1.0 2.6 1.8 1.0 1.2 1.1 B

6 0.6 A 0.8 B 0.7 1.6 2.2 1.9 0.2 3.0 1.6 B

Mean 0.3 1.0 1.0 1.7+ 0.8 3.4**

P



y PMeans in columns at 0 days post-treatment followed by the same letter do not differ at p < 0.05 according to LSD test. Means in

columns at 43 days post-treatment followed by the same letter do not differ at p < 0.1 according to LSD test. 29

P

zP +, *, ** indicate significant differences between solarized and non-solarized at p < 0.10, 0.05, and 0.01, respectively. No symbol


Table 2-12. Collembolans during 2004 summer solarization experiment.

Collembola per 200 cm P

3P soil

________________________________________________________________________________________________


____________________ ______________________ ______________________ _____________________

DurationP


2 0.0 2.8 1.4 0.6 AP

yP 1.0 B 0.8 2.4 4.2 3.3 6.0 6.0 6.0

4 0.2 0.2 0.2 0.0 A 1.2 B 0.6 1.6 2.0 1.8 7.0 10.8 8.9

6 0.4 0.6 0.5 1.0 A 17.0 A+P

z P 9.0 1.6 4.2 2.9 7.4 14.8 11.1

Mean 0.2 1.2 0.5 6.4 1.9 3.5 6.8 10.5

P




P

zP + indicates significant differences between solarized and non-solarized at p < 0.10. No symbol indicates no significant differences. 3

0

Table 2-13. Mites (including non-oribatid and oribatid mites) during 2003 summer solarization experiment.

Mites per 200 cmP

3P soil

__________________________________________________________________________________________


________________________ _________________________ ______________________________

DurationP


2 1.6 1.0 1.3 4.4 4.4 4.4 ABP

yP 3.0 A 7.2 A*P

zP 5.1

4 1.8 2.2 2.0 6.4 8.2 7.3 A 2.0 A 1.4 B 1.7

6 0.4 3.8 2.1 2.0 4.6 3.3 B 3.6 A 2.8 B 3.2

Mean 1.3 2.3 4.3 5.7 2.9 3.8

P




P

zP * indicate significant differences between solarized and non-solarized at p < 0.05. No symbol indicates no significant difference. 3

1

32

Table 2-14. Mites during 2004 summer solarization experiment.

Mites per 200 cmP

3P soil

________________________________________________________________________________________________


______________________ ______________________ ______________________ ________________________

DurationP


2 0.0 0.6 0.3 0.2 0.2 0.2 0.6 0.0 0.3 1.8 3.2 2.5 BP

y

4 0.0 2.6 1.3 0.0 2.2 1.1 1.2 0.0 0.6 1.6 3.8 2.7 B

6 0.0 1.4 0.7 0.2 1.8 1.0 0.2 0.2 0.2 3.0 7.4 5.2 A

Mean 0.0 1.5*P

zP 0.1 1.4+ 0.7 0.1* 2.1 4.8*

P




P

zP +, * indicates significant differences between solarized and non-solarized at p < 0.10, and 0.05, respectively. No symbol indicates no

significant differences.

CHAPTER 3

WEED POPULATION DYNAMICS AFTER SUMMER SOLARIZATION

Introduction

Solarization is an effective method for the management of soil-borne pests,

including weeds. Clear plastic mulch allows for the transmission of solar radiation that

heats the soil to lethal or near-lethal temperatures (30-60°; Katan, 1981). Several

mechanisms of weed control by solarization have been proposed. These mechanisms

include direct thermal damage to seeds, germinating seeds, and shoots; morphological

changes in shoots and other plant organs; breaking of dormancy and germination at

greater depths; an imbalance of gases in soil; and indirect effects on soil microorganisms

that might be seed pathogens (Rubin and Benjamin, 1984; Elmore, 1991; Chase et al.,

1998).

Several studies have focused on thermal damage to a variety of weed species.

Rubin and Benjamin (1984) found that weed species responded differently to exposure to

temperatures from 30-90°C. Redroot pigweed (Amaranthus retroflexus L.) emergence

was reduced when exposed to 30 minutes of 50°C; but even at 90°C sweet clover

(Melilotus sulcatus Desf.) was not affected (Rubin and Benjamin, 1984). Rhizomes of

bermudagrass (Cynodon dactylon (L.) Pers.) were killed when exposed to 30 minutes of

temperatures above 40°C, but tubers of purple nutsedge (Cyperus rotundus L.) required

temperatures above 60°C to show any reduction in viability (Rubin and Benjamin, 1984).

Further study of thermal effects on velvetleaf (Abutilon theophrasti Medik) revealed that

shorter, more frequent exposure to high temperatures, which would mimic the diurnal

33

34

heating produced by solarization, reduced germination rates compared to a single

exposure (Horowitz and Taylorson, 1983).

The nutsedges (Cyperus spp.) have been of particular concern in solarization

research, in part because they are among the world’s worst weeds (Holm et al., 1977).

Studies have focused on nutsedge control by various types of solarization plastic

(Patterson, 1998; Chase et al., 1998; Chase et al., 1999). Purple nutsedge shoots were

able to penetrate opaque polyethylene mulches and some clear mulches of various

thicknesses and plastic types, but penetration was reduced when a 5-10 mm space was

created between plastic and soil (Chase et al., 1998). Solarization treatment using clear

mulch caused a morphological change in the nutsedge shoots that greatly reduced

penetration and caused the shoots to be scorched under the plastic (Chase et al., 1998).

Laboratory exposure to 50°C for 6 hours daily resulted in 100% mortality of nutsedge

tubers, and exposure to 45°C reduced emergence of nutsedge shoots (Chase et al., 1999).

This information can be applied to a management plan that includes solarization as a

method of killing the shoots and depleting the plant reserves in the nutsedge tubers.

This study was designed to examine the effects of solarization on weed

populations and a variety of summer weed species, including nutsedges, hairy indigo

(Indigofera hirsuta L.), carpetweed (Mollugo verticillata L.), goosegrass (Eleusine indica

(L.) Gaertn.), and southern crabgrass (Digitaria ciliaris (Retz.) Koeler). The main

hypothesis was that as solarization treatment duration increased, weed coverage would

decrease. We further hypothesized that the residual effects (up to 67 days post-treatment)

would vary based on initial treatment duration. Additionally, the effects of solarization

were expected to vary by individual plant species.

35

Methods

The experiment was conducted during the summers of 2003 and 2004 at the

University of Florida Plant Science Research and Education Unit in northern Florida. The

soils in the experimental area were hyperthermic, uncoated, typic Quartzipsamments of

the Candler series with a 0 to 5% slope (Thomas et al., 1979). Average soil pH of the site

was measured as 5.9. The measured soil texture was 95% sand, with 3% clay, and 2%

silt. A crimson clover (Trifolium incarnatum L. ‘Dixie’) cover crop was planted during

the winter seasons prior to each experiment and disked two days before the first plots

were constructed.

The experiment was designed as a split-plot with duration of treatment as the main

effect and solarization as the sub effect. Five replicates were arranged in a randomized

complete block on the main effect. The experimental plots were raised beds 6 m long, 1

m wide, and 20 cm high. The soil was moistened by overhead irrigation if it was not

sufficiently moist before the application of solarization plastic. The solarization plastic

was a single layer of clear, 25-µm-thick, UV-stabilized, low-density polyethylene mulch

(ISO Poly Films, Inc., Gray Court, SC). Application of solarization treatments and non-

solarization control treatments of 2, 4, and 6 week durations occurred during July and

August of 2003 and 2004. Temperature data loggers (Watch Dog P

RP Model 425, Spectrum

Technologies, Inc., Plainfield, IL) were inserted into the soil at depths of 5, 10, and 15 cm

in 6-week solarized and non-solarized plots, 4-week solarized plots, and 2-week solarized

plots in 2003 and 2004. The treatments started on sequential dates and concluded in mid-

August both years.

36

Following solarization treatment, all plastic was removed and the middle 2.0 m of

each 6.0-m plot was planted with 5-week old okra (Hibiscus esculentus L. ‘Clemson

spineless’) seedlings and used to monitor plant nutrition, soil chemistry, and insect, and

nematode populations (Seman-Varner, Chapters 2 and 4). On either side of the okra

plants, a 1.0-m P

2P subplot was established to monitor weed populations in the experimental

plots. Weed populations were monitored at 2-week intervals following solarization

treatment and plastic removal. The Horsfall-Barratt scale (Horsfall and Barratt, 1945)

was used to determine the percent ground coverage by weeds in each plot on a scale from

1 to 12, where 1 = 0%; 2 = 0-3%; 3 = 3-6%; 4 = 6-12%; 5 = 12-25%; 6 = 25-50%; 7 =

50-75%; 8 = 75-88%; 9 = 88-94%; 10 = 94-97%; 11 = 97-100%; 12 = 100% of ground

area covered. Individual plants were counted by species and then totaled for each plot.

When plots reached 100% coverage, individual plants were no longer counted. A separate

category for individual seedling counts was used for small seedlings that were visible but

not easily identified and not a contributing factor to coverage. Final weed biomass was

collected from a 0.25-mP

2 Psection of the weed subplot and weighed fresh. During the first

field trial, exposed trays [54.6 cm x 26.7 cm (21.5 x 10.5 in)] of sterilized soil were

placed at each corner and each mid-point of the experimental area to determine how

much of the weed seed population was dispersed by wind.

Weed coverage based on HB ratings, total and individual species plant populations,

and weed biomass were compared among durations and between solarized and non-

solarized treatments using analysis of variance. If significant differences were detected

among duration treatments, means were separated using a Least Significant Difference

37

test at the α= 0.05 level. All data were analyzed using MSTAT-C software (Michigan

State University, East Lansing, MI).

During the field experiment in 2004, several events occurred that may have

influenced the experiments and data. On 6 August 2004, herbicide [Roundup (propan-2-

amine)] was applied accidentally to some of the plots. The control treatments were

exposed while the solarized treatments remained under plastic. The error was

immediately recognized and the areas affected were rinsed with water. Additionally three

hurricanes occurred during the season that caused an increase in total rainfall over the

experimental period [94 cm (37 in)] to more than twice that in the previous season [38 cm

(15 in); Florida Automated Weather Network, 2005]. Further weather data was obtained

from the Florida Automated Weather Network, including air temperature, precipitation,

and solar radiation (Florida Automated Weather Network, 2005).

Results

Weed Cover

Significant interactions (p < 0.05) between solarization treatment and duration

occurred on all sampling dates in 2003. The Horsfall-Barratt scale rating (HB) for weed

coverage was significantly lower (p < 0.05) in solarized treatments than in non-solarized

treatments on every sampling date in 2003 (Table 3-1). The HB ratings on the first and

second sampling dates (0 and 14 days post-treatment) were between 1.0 and 2.0 in

solarized plots and showed no significant differences among the three duration

treatments. The coverage ratings in non-solarized treatments ranged between 2.0 and 10.0

and were higher than in solarized treatments (p < 0.01). Among non-solarized durations

on both sampling dates, weed coverage was up to 35% and 72% lower in the 2-week

treatment than in the 4- or the 6-week treatments, respectively (p < 0.05) and the 6-week

38

treatment was up to 56% higher than the 4-week treatment (p < 0.05). At 28 days post-

treatment, weed coverage in solarized treatments was 54% to 73% lower than non-

solarized treatments (p < 0.05). The HB rating in 6-week non-solarized treatments was

significantly higher than the 2-week or the 4-week non-solarized treatment (p < 0.05). At

43 days post-treatment, the HB rating in solarized plots remained significantly lower than

non-solarized plots by 35% to 68% (p < 0.05). In non-solarized duration treatments, the

HB rating in the 6-week treatment was 50% higher than that in the 2-week treatment (p <

0.05). By the final sampling date (56 days post-treatment), weed coverage in the solarized

treatments remained significantly lower than non-solarized treatments (p < 0.01). There

were no significant differences among durations in solarized or non-solarized treatments.

When solarization plastic was removed in 2004 (0 days post-treatment), the HB

ratings were significantly higher in the 6-week non-solarized plots than in any of the

other non-solarized or solarized duration treatments (p < 0.05; Table 3-2). There was no

difference between the solarized and non-solarized treatments of 2-week or 4-week

durations. On the next 3 sampling dates (11, 25, and 39 days post-treatment), HB ratings

were 44%, 37% and 24% lower in solarized treatments compared to non-solarized

treatments, respectively. On the last two sampling dates (53 and 67 days post-treatment),

significantly reduced HB ratings continued in solarized treatments (p < 0.1). On these

dates, significant differences occurred among duration means; the 2-week treatments had

the highest HB rating and the 4-week had the lowest, while the 6-week treatments were

not different from either of the other durations.

Total Weed Populations

In 2003, total weed populations were significantly lower in solarized plots than in

non-solarized plots at 0 and 14 days post-treatment, by as much as a factor of 200 (p <

39

0.05; Table 3-3). Among non-solarized treatments, there were significantly higher

numbers of weeds per plot in the 6-week treatment than in the 4- or the 2-week

treatments (p < 0.05). Some non-solarized plots reached 100% coverage by 28 days post-

treatment, limiting the comparison of solarized plots with non-solarized plots. There were

no significant differences in total weed populations among solarization duration

treatments at 28, 43, and 56 days post-treatment (p > 0.1).

In 2004, total weed populations were initially reduced by 72% in solarized plots

compared to non-solarized plots at 0 days post-treatment (Table 3-4). There were no

significant differences (p > 0.10) in weed population on any of the subsequent sampling

dates (11, 25, 39, and 53 days post-treatment). Means of the duration treatments were

significantly different at 39 and 53 days post-treatment, with the 4-week treatment having

the lowest total weed population, and the 6-week having the highest total weed

population (p < 0.05). Plots of solarized and non-solarized treatments reached 100%

coverage by 67 days post-treatment, and consequently individual plants were no longer

counted.

Individual Weed Species Populations

Based on population levels of individual weeds in 2003, the most dominant

species was hairy indigo. At 0 days post treatment, hairy indigo was eliminated in all

solarized treatments and significantly lower than all non-solarized treatments (p < 0.05;

Table 3-5). In non-solarized treatments, the 6-week plots had more than 6 times the

number of hairy indigo plants than in the 2-week plots and almost 3 times the number in

the 4-week plots (p < 0.05). By 14 days post-treatment, hairy indigo appeared in the 2-

week solarized treatment in equal numbers to those in the 2-week non-solarized

treatment. However, hairy indigo was completely suppressed in the 6-week solarized

40

treatments and only very few appeared in the 4-week solarized treatments. Both 4- and 6-

week solarized treatments remained significantly lower than non-solarized treatments (p

< 0.05). The 6-week non-solarized treatment had more than twice the number of hairy

indigo plants than the 4-week non-solarized treatment and more than 5 times the number

in the 2-week non-solarized treatment (p < 0.05). Once non-solarized plots reached 100%

coverage (28 days post-treatment), hairy indigo numbers were significantly lower in 4-

and 6-week solarization treatments than in 2-week solarization treatments.

Purple nutsedge was also an important species, based on the number of stems

contributing to the total weed count (Table 3-6). Initially (0 days post-treatment),

solarization treatments almost eliminated nutsedge plants, and significantly reduced the

number of nutsedge sprouts in 4-week and 6-week treatments when compared to the non-

solarized treatments (p < 0.05). At 14 days post-treatment, nutsedge sprouts remained

low in solarized plots but were low in non-solarized plots as well, showing no significant

difference from solarized. Once non-solarized plots reached 100% coverage by total

weeds, there were no significant differences among durations of solarization treatment.

In 2004, weed density was dominated by goosegrass (Eleusine indica), southern

crabgrass (Digitaria ciliaris), carpetweed (Mollugo verticillata), hairy indigo, and purple

nutsedge. There were no significant effects of solarization treatments on any of the

individual weed species during the 2004 field season.

Weed Biomass

Total weed biomass was reduced by 90% in solarized treatments compared to non-

solarized treatments in 2003 (Table 3-7). Hairy indigo biomass was reduced by 98% in

solarized treatments. Hairy indigo made up 75% of the total weed biomass in non-

solarized plots. In solarized treatments, hairy indigo accounted for 16% of the total

41

biomass. Bermudagrass was the dominant species by weight in solarized plots and

comprising nearly 30% of the total weed biomass. Carpetweed comprised the next largest

proportion of total biomass with 18%.

In 2004, there was no significant difference in total weed biomass between

treatments and no significant trend in biomass for individual species. Goosegrass was the

dominant species in both solarized and non-solarized treatments, making up 62% and

45% of the total weed biomass in each treatment, respectively. Lambsquarter

(Chenopodium album L.) was also very important and comprised 10% of weed biomass

in solarized plots and 25% in non-solarized plots.

Wind-dispersed Seed

The wind-dispersed seed traps did not contain any germinated plants, indicating

that all weeds were germinated from the soil seed bank.

Discussion

Solarization effectively reduced weed coverage based on the HB ratings and weed

population counts in 2003. There were no significant differences in weed coverage

among durations of solarization treatment for the first 28 days post-treatment. All five 6-

week non-solarized plots reached 100% coverage by 28 days post-treatment in 2003. By

the conclusion of the experiment, an additional two 4-week non-solarized plots reached

100% coverage, while no solarized plots ever reached 100% coverage.

In 2004, HB ratings were significantly lower in solarized treatments than in non-

solarized treatments, despite the application of herbicide to the control plots. The

herbicide decreased weed coverage in non-solarized plots, evident in the delay in the time

it took to reach 100% weed coverage and the lower average maximum HB rating at the

final sampling, which was 9.3 in 2004 and 10.9 in 2003. In contrast to the 2003

42

experiment, 67 days elapsed in 2004 (vs. 28 days in 2003) before 100% coverage was

reached in any plots, which included three 6-week non-solarized plots, one 4-week non-

solarized plot, two 2-week non-solarized plots, and one 2-week solarized plot. The

inconsistency can be attributed to the application of herbicide to control plots. Most

importantly, this reduction of weeds in control plots made it difficult to demonstrate

significant differences between solarized and non-solarized treatments in 2004.

After solarization treatment in 2003, total weed counts were reduced to almost 0

and maintained a reduction of 80% or more compared to non-solarized plots. Maximum

soil temperatures at 10 cm deep in solarized plots reached 40°C or more on 20 days of the

41 days of solarization treatment in 2003 (Figure 3-1). These high-temperature days

would have damaged seeds and emerging plants at this depth. Although several control

plots reached 100% coverage at 28 days post-treatment, which limited effective counting

of weeds and analysis of variance between solarized and non-solarized treatments,

average total weed count for each duration in solarized plots remained very low, with

fewer than 7 stems mP

-2P. There were no differences among durations of solarization based

on total weed count, suggesting that all solarization treatment durations were effective in

decreasing weed emergence. The maximum soil temperatures indicate an equal

distribution of high temperatures during the first 4 weeks of the treatment, or until mid-

August (Figure 3-1). The final two weeks of the experimental period show a general

decrease in the maximum temperatures reached each day, while still maintaining

temperatures well above 35°C.

In 2004, total weed counts were reduced initially by solarization treatment. Soil

temperatures were sufficiently high (42°C average daily maximum) to damage weed seed

43

in the soil (Figure 3-2). The application of herbicide to the non-solarized treatments

caused an unintentional reduction in total weed counts for those control plots, influencing

the total count on subsequent sampling dates. The last two sampling dates (39 and 53

days post-treatment) showed a significant trend where the 4-week treatment means were

lower than the 6-week treatment means but not different from the 2-week treatment

means, likely due to the herbicide application. The herbicide application interfered with

the accurate assessment of solarization treatment effects by reducing weed populations in

non-solarized plots.

Individual species counts from 2003 illustrated that the two dominant species, hairy

indigo and purple nutsedge, were variably affected by solarization treatments. Other

studies also have shown that high-temperature affects, like those caused by solarization

or other methods, vary with species and depth of the seed (Horowitz et al., 1983;

Standifer et al., 1984; Egley, 1990). Hairy indigo was reduced by solarization and

continued to be affected by solarization of 4-and 6-week durations, up to 56 days post-

treatment. Purple nutsedge was initially reduced by 4- and 6-week solarization

treatments. However, at 14 days post-treatment, numbers of purple nutsedge had dropped

to < 1.5 per m P

2P in the non-solarized plots. The reduction in purple nutsedge may have

been due to competition with the larger plants of hairy indigo, which quickly became the

dominant weed in those plots. Therefore, at 14 days post-treatment and beyond, there

were no significant differences among durations of solarization treatment on nutsedge

populations. No weed species were individually affected by solarization or duration

treatment in 2004, again due to herbicide application.

44

The 90% reduction in weed biomass in solarized treatments was another indicator

that solarization was effective in reducing weed populations, even up to 56 days post-

treatment. Solarization was also effective in reducing hairy indigo, the most dominant

weed during 2003. There were no differences in weed biomass for the 2004 field season,

primarily due to the application of herbicide early in August.

Conclusion

Solarization was effective in reducing weed coverage, density, and biomass in

2003. Increasing the duration of solarization treatment from 2 to 6 weeks did not

significantly affect coverage ratings or total weed density. Studies have shown effective

control of some winter and summer annual weed species with as little as 1-2 weeks

solarization (Egley, 1983; Horowitz et al., 1983; Elmore, 1991). However, the effect of

both 4- and 6-week treatments on Florida weed populations appears to continue beyond

that of the 2-week treatment, in which weed populations begin to recover at an earlier

date. Durations of 4 and 6 weeks also more effectively reduced individual weed species

in Florida, specifically hairy indigo and purple nutsedge.

Further study on weed populations after plots reach 100% coverage may add to

our understanding of the lasting effects of solarization. An in-depth study on individual

species populations (e.g., hairy indigo, nutsedges, goosegrass, and southern crabgrass)

would further our understanding of which species are more resistant to the effects of high

temperature and how seeds in the soil seed bank resist damage or recover from

solarization treatment.

Table 3-1. Horsfall-Barratt ratings for weed coverage during 2003 summer solarization experiment.

____________________________________________________________________________________________________________

Horsfall-Barratt ratingP

wP by sampling date

______________________________________________________________________________________________________

0 days 14 days 28 days 43 days 56 days

__________________ __________________ __________________ ___________________ __________________

Dur P

xP Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean

2 1.2 AP

y P 2.0 C**P

zP 1.6 1.8 A 3.4 C** 2.6 2.8 A 6.1 B* 4.4 5.2 A 8.0 B* 6.6 6.4 A 9.5 A** 7.9

4 1.0 A 3.1 B*** 2.0 1.9 A 5.1 B*** 3.5 2.1 A 7.4 B*** 4.7 4.1 A 9.6 AB** 6.8 4.9 A 11.1 A*** 8.0

6 1.2 A 7.1 A*** 4.1 1.6 A 9.3 A*** 5.5 3.2 A 12.0 A*** 7.6 3.9 A 12.0 A*** 8.0 4.5 A 12.0 A** 8.2

Mean 1.1 4.0 1.8 5.9 2.7 8.5 4.4 9.8 5.2 10.9

P

w PRated on 1 (0% of plot area covered) to 12 (100% of plot area covered) scale (Horsfall and Barratt, 1945) for percentage of plot area

covered by weeds. P

x PDuration (Dur) of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 12 August 2003; post-treatment times

measured after this date (X days). 45

P


P

zP *, **, *** indicate significant differences between solarized and non-solarized at p < 0.05, 0.01, and 0.001, respectively.

Table 3-2. Horsfall-Barratt ratings for weed coverage during 2004 summer solarization experiment.

____________________________________________________________________________________________________________

Horsfall-Barratt ratingP

wP by sampling date

______________________________________________________________________________________________________

0 days 11 days 25 days 39 days 53 days 67 days

__________________ _____________ ______________ ______________ _____________ ________________

Dur P

xP Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean

2 2.0 AP

y P 2.1 B 2.1 2.2 2.8 2.5 3.4 4.7 4.1 6.4 6.7 6.6 7.6 7.4 7.5 A 10.0 10.0 10.0 A

4 2.0 A 2.9 B 2.5 2.0 3.8 2.9 2.6 4.3 3.5 3.7 5.4 4.6 4.4 6.4 5.4 B 5.4 8.2 6.8 B

6 2.0 A 6.9 A**P

z P 4.5 2.8 5.8 4.3 3.3 5.8 4.6 4.4 6.8 5.6 5.6 7.8 6.7 AB 6.8 9.6 8.2 AB

Mean 2.0 4.0 2.3* 4.1 3.1** 4.9 4.8* 6.3 5.9+ 7.2 7.4+ 9.3

P

w PRated on 1 (0% of plot area covered) to 12 (100% of plot area covered) scale (Horsfall and Barratt, 1945) for percentage of plot area

covered by weeds. P

x Duration (Dur) of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 11 August 2004; post-treatment times

measured after this date (X days). 46y

Means in columns followed by the same letter do not differ at p < 0.05 according to LSD test; no letters indicate no differences at p

< 0.05. z +, *, ** indicate significant differences between solarized and non-solarized at p < 0.10, 0.05, and 0.01, respectively. No symbol


Table 3-3. Weed population (number of plants or stems) during 2003 summer solarization experiment.

____________________________________________________________________________________________________________

Number of weed plants or stems per m2

______________________________________________________________________________________________

0 days 14 days 28 daysz 43 days 56 days

____________________ ______________________ ________ _________ ________

Durationw Sol Non Mean Sol Non Mean Sol Sol Sol

2 0.2 Ax

16 B**y 8.1 1.9 A 10.4 B** 6.2** 5.8 A 6.6 A 6.2 A

4 0.0 A 58.6 B** 29.3 0.6 A 10.5 B** 5.6 2.2 A 2.5 A 2.6 A

6 0.8 A 200.8 A* 100.8 1.2 A 25.1 A*** 13.2 6.5 A 4.5 A 6.0 A

Mean 0.3 91.8 1.2 15.3 w

Duration of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 12 August 2003; post-treatment times measured

after this date (X days). x Means in columns followed by the same letter do not differ at p < 0.05 according to LSD test.

y *, **, *** indicate significant differences between solarized and non-solarized at p < 0.05, 0.01, and 0.001, respectively. 4

7z treatment plots reached 100% coverage by 28 days post-treatment, therefore only solarization treatments were examined for total

weed counts.

Table 3-4. Weed population (number of plants or stems) during summer solarization experiment 2004.

Number of weed plants or stems per m2

______________________________________________________________________________________________________

0 days 11 days 25 days 39 days 53 daysz

__________________ ________________ ________________ _________________ ____________________

Durationw Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean

2 20.2 41.4 30.8 27.2 16.4 21.8 29.6 18.6 24.1 30.2 16.0 23.1 ABx 26.4 16.4 21.4 AB

4 36.0 68.6 52.3 14.2 18.0 16.1 10.0 12.2 11.1 11.4 10.0 10.7 B 10.4 13.4 11.9 B

6 8.8 121.2 65.0 11.2 25.2 18.2 16.2 20.6 18.4 31.4 26.2 28.8 A 44.0 30.4 37.2 A

Mean 21.7 77.1+y 17.5 19.9 18.6 17.1 24.3 17.4 26.9 20.1

w Duration of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 11 August 2004; post-treatment days measured

after this date (X days). x Means in columns followed by the same letter do not differ at p < 0.05 according to LSD test; no letters indicate no differences at p

< 0.05. 48

y + indicates significant differences between solarized and non-solarized at p < 0.10. No symbol indicates no significant difference.

z solarized and non-solarized treatment plots reached 100% coverage by 67 days post-treatment, therefore total weeds were not

counted at 67 days.

Table 3-5. Number of hairy indigo (Indigofera hirsuta) plants during 2003 summer solarization experiment.

___________________________________________________________________________________________________________

Number of hairy indigo plants per m2

______________________________________________________________________________________________


____________________ ______________________ ________ _________ ________


2 0.0 Ax

12.2 B*y 6.1 1.7 A 2.3 B 2.0 3.0 A 2.8 A 2.8 A

4 0.0 A 27.2 B** 13.6 0.4 A 4.8 B* 2.6 1.0 B 1.0 B 1.0 B

6 0.0 A 78.0 A** 39.0 0.0 A 12.4 A** 6.2 1.0 B 0.6 B 0.6 B

Mean 0.0 39.1 0.7 6.5 w



< 0.05. 49y

*, ** indicate significant differences between solarized and non-solarized at p < 0.05 and 0.01, respectively. z treatment plots reached 100% coverage by 28 days post-treatment, therefore only solarization treatments were examined for hairy

indigo plant counts.

Table 3-6. Number of purple nutsedge (Cyperus rotundus) stems during 2003 summer solarization experiment.

____________________________________________________________________________________________________________

Number of purple nutsedge stems per m2

______________________________________________________________________________________________


____________________ ______________________ ________ _________ ________


2 0.2 Ax

1.2 B 0.7 0.0 1.4 0.7 0.9 1.1 0.8

4 0.0 A 2.6 B*y 1.3 0.1 0.9 0.5 0.6 0.2 0.4

6 0.6 A 11.2 A** 5.9 1.0 1.1 1.1 0.0 0.6 1.4

Mean 0.3 5.0 0.4 1.1 w



< 0.05. 50y

*, ** indicate significant differences between solarized and non-solarized at p < 0.05 and 0.01, respectively. z treatment plots reached 100% coverage by 28 days post-treatment, therefore only solarization treatments were examined for total

weed counts.

Table 3-7. Total weed biomass at conclusion of summer solarization experiments during 2003 (56 days post-treatment) and 2004 (67

days post-treatment).

Fresh weight (g) of total weed biomass per 0.25 m2

______________________________________________________________________

2003 2004

_______________________ _________________________

Durationy Sol Non Mean Sol Non Mean

2 50.9 259.2 155.0 130.2 57.0 93.6

4 23.2 226.7 124.9 85.7 38.3 62.0

6 65.8 849.7 457.7 43.1 114.4 78.8

Mean 46.6 445.2**z 86.3 69.9

y Duration of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 12 August 2003 and 11 August 2004.

z ** indicates significant differences between solarized and non-solarized at p < 0.01.

51

20.0

25.0

30.0

35.0

40.0

45.0

50.0

1 5 9 13 17 21 25 29 33 37 41

Number of days of solarization treatment (days)

Maxim

um

soil

tem

peratu

re a

t 10 c

m (

C)

non-solarized

solarized

52

Figure 3-1. Daily maximum soil temperatures at 10 cm depth during 6-week solarized and non-solarized treatments in 2003.

53

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

1 5 9 13 17 21 25 29 33 37

Number of days of solarization treatment (days)

Maxim

um

soil

tem

per

atu

re a

t 10 c

m (

C)

non-solarized

solarized

Figure 3-2. Daily maximum soil temperatures at 10 cm depth during 6-week solarized and non-solarized treatments in 2004.

CHAPTER 4

SOIL NUTRIENT AND PLANT RESPONSES TO SOLARIZATION IN AN

AGROECOSYSTEM UTILIZING AN ORGANIC NUTRIENT SOURCE

Introduction

Soil solarization has been used to successfully manage soil-borne pests including

fungal and bacterial pathogens, nematodes, and weeds. Use of solarization has increased

crop yield at sites with various pest problems (Davis, 1991; Elmore, 1991; McGovern

and McSorley, 1997). Solarization may cause an increased growth response in plants not

only due to reductions in soil pathogens but also due to changes in chemical or physical

properties of the soil (Chen and Katan, 1980; Stapleton et al., 1985).

Studies on solarization effects on mineral nutrients in soil have had mixed results.

One study involving eight soil types found large increases in the concentration of NO B3B

-

and NH B4B

+, which may have been the result of increased organic matter in the soil (Chen

and Katan, 1980). In the same study, concentrations of Ca, K, Na, and Mg, and soil

electrical conductivity increased consistently as well, while pH and P concentration

remained the same or changed inconsistently (Chen and Katan, 1980). Stapleton et al.

(1985) found similar results and confirmed the increase in NO B3B

-, NH B4B

+, Ca, and Mg

concentrations with solarization in four soil types in California (Stapleton et al., 1985).

Contrary to the findings of Chen and Katan (1980), Stapleton et al. (1985) found an

increase in P concentration, no change in K concentration, and inconsistent results with

electrical conductivity. The discrepancies were likely due to differences in soil types,

sampling depth, and assay procedures (Stapleton et al., 1985).

54

55

Of the studies examining the effects of solarization on soil chemistry and physical

properties, few have included an examination of these effects on plant nutrition. Tomato

(Lycopersicon esculentum Mill.) seedlings grown in solutions from solarized and non-

solarized soils and in pure water exhibited significant increases in plant height, leaf

length, and whole plant dry weight when grown in solution from solarized soil (Chen and

Katan, 1980). In another study, a 33% increase in fresh weights of Chinese cabbage

(Brassica rapa L. ‘Lei Choi’) was observed in solarized versus untreated soil, with a

further increase of 28% if solarized treatments were fertilized (Stapleton et al., 1985).

The study also included plant tissue analysis but found no significant differences in

nutrient concentrations between treatments (Stapleton et al., 1985). The authors

concluded that the increases in plant growth may be attributed to a combination of

pathogen reduction, increases in available soil nutrients, and other ecological factors

caused by solarization (Stapleton et al., 1985).

The primary objective of this study was to determine if duration of solarization

treatment had an effect on soil mineral nutrients, soil properties, and plant leaf tissue

nutrients. An additional objective was to determine the effects of solarization on plant

nutrition in a system that utilized an organic nutrient source.

Methods

This experiment was conducted at the University of Florida’s Plant Science

Research and Education Unit (PSREU) near Citra, Florida, during the summers of 2003

and 2004. The soil at the study site was a hyperthermic, uncoated typic

Quartzipsamments of the Candler series with a 0 to 5% slope (Thomas et al., 1979).

Measured soil texture was 95% sand, 2% silt, and 3% clay. Measured soil pH ranged

from 5.5 to 6.0 with an average of 5.9. The field was prepared with a crimson clover

56

(Trifolium incarnatum L. ‘Dixie’) cover crop during the winter season and disked two

days before beds were constructed.

The experiment was a split-plot design in which the main effect was duration of

treatment and the sub-effect was solarization. Five replicates were arranged in a

randomized complete block design on the main effect. Each experimental plot was a

raised bed 6 m long, 1 m wide, and 20 cm high. The soil was moistened by overhead

irrigation if it was not sufficiently moist before plastic application. The solarization

treatment was installed using a single layer of clear, 25-µm-thick, UV-stabilized, low-

density polyethylene mulch (ISO Poly Films, Inc., Gray Court, SC). Solarization

treatments and non-solarization control treatments of 2, 4, and 6 week durations were

conducted during July and August. The 2003 and 2004 treatments began on sequential

dates and concluded on 12 August 2003 and 11 August 2004, respectively.

Six soil cores 2.5 cm in diameter and 15 cm deep were collected from each plot at 0

days post-treatment in 2003 and 4 days post-treatment in 2004. The samples were air

dried and sieved to pass a 2-mm stainless steel screen. Nitrogen concentration was

determined for soil samples using a modified micro-Kjeldahl procedure (Bremner, 1965)

and further modified as follows (Gallaher, personal communication, 2005). A 2-g sample

was placed into a 100-ml Pyrex test tube followed by the addition of 3.2-g salt catalyst

mixture (9:1 of K B2BSO B4B : CuSO B4B) and then 10 ml H B2BSO B4B. Samples were mixed using a

vortex mixer. This was followed by the addition of two 1-ml increments of H B2BO B2B after

tubes were placed in the aluminum block digester (Gallaher et al., 1975). Pyrex funnels

were placed on the top of each test tube to help conserve acid and increase reflux action

during digestion for 3.5 hours at 375°C. Tubes were cooled and mixed with the vortex

57

mixer again while diluting with deionized water to 75 ml volume. Solutions were

transferred to plastic storage bottles. Subsamples of solutions were placed in small test

tubes and analyzed colorimetrically using a Technicon Autoanalyzer II. Nitrogen levels

were recorded on a graphics printout recorder. The unknown readings were charted

against known standards and concentrations determined using a simple regression model

computer program (Gallaher, personal communication, 2005).

Mineral concentrations in solution were first extracted from the soil using the

Mehlich I double-acid method (Mehlich, 1953). Soil Ca, Mg, Cu, Fe, Mn, Zn were

determined by atomic absorption spectrometery, soil K and Na by atomic emission

spectrometry, and soil P by colorimetry. Soil pH was measured at a 2:1 water to soil ratio

using a glass electrode (Hanlon et al., 1994). Mechanical analysis was used to determine

percent sand, silt, and clay using a soil hydrometer method (Bouyoucos, 1936; Day,

1965). Percent organic matter was determined using the Walkley-Black method

(Walkley, 1935; Allison, 1965). Cation exchange capacity was determined by the

summation method of relevant cations (Hesse, 1972; Jackson, 1958).

Okra (Hibiscus esculentus L. ‘Clemson spineless’) seeds were planted and

germinated in a growth room, then moved to a greenhouse for maturation. The seedlings

were watered and fertilized with a 20-20-20 (N-P B2BO B5B-K B2BO) mix as needed for 5 weeks.

One week after the conclusion of solarization treatments, okra seedlings were planted in

the experimental plots. An organic fertilizer consisting of green cowpea (Vigna

unguiculata (L.) Walp. ‘Iron Clay’) hay was applied on the soil surface to the area

immediately around the okra seedlings at a rate of 3.5 kg m-2

. This hay was obtained from

a cowpea cover crop growing in an adjacent field, which was cut at the early bloom stage,

58

and applied fresh to the plots on the same day it was cut. The cowpea was analyzed for

nutrient concentration from samples collected at the time of application. The okra was

harvested 6 weeks after planting, at early flowering. Dry whole plant biomass was

measured and the youngest, fully expanded okra leaves were collected for nutrient

analysis.

For the nutrient analysis of both okra and cowpea plant material, plant material was

weighed, dried, and ground and solutions were prepared using the Mehlich I double-acid

method (Mehlich, 1953). Nitrogen concentration was determined using a method similar

to that used for the soil samples, except that a 100-mg sample was used and boiling beads

were added to the samples before being placed on the aluminum block digester (Bremner,

1965; Gallaher et al., 1975; Gallaher, personal communication, 2005). Leaf tissue

concentrations of Ca, Mg, Cu, Fe, Mn, and Zn were determined by atomic absorption

spectrometery, K and Na by atomic emission spectrometry, and P by colorimetry.

Okra biomass, okra leaf nutrient concentration, and extractable soil nutrient data

were compared among durations and between solarized and non-solarized treatments

using analysis of variance (ANOVA). If significant differences were detected among

duration treatments, means were separated using a Least Significant Difference test at the

α = 0.05 level. All data were analyzed using MSTAT-C software (Michigan State

University, East Lansing, MI).

During the field experiment in 2004, several events occurred that may have

influenced the results. On 6 August 2004, herbicide [Roundup (propan-2-amine)] was

applied accidentally to some of the plots. The control treatments were exposed while the

solarized treatments remained under plastic. The error was immediately recognized and

59

the areas affected were rinsed with water. Further, three hurricanes occurred during the

season that caused an increase in total rainfall over the experimental period [94 cm (37

in)] to more than twice that in the previous season [38 cm (15 in], which may have

caused Pythium sp. infection of the okra plants (Florida Automated Weather Network,

2005). The hurricanes also brought high winds that damaged many of the okra plants,

decreasing the number of replicates in the experimental plots.

Results

Soil Mineral Nutrients

In the 2003 experiment, a significantly (p < 0.05) higher concentration of Mn (mg

kg-1

) was found in the soil of solarized treatments (Table 4-1). CEC (meq 100g-1

) was

also higher (p < 0.05) in solarized treatments (Table 4-2). Zinc (mg kg-1

) and organic

matter (%) were significantly lower in solarized treatments (p < 0.10). The concentration

of Na was reduced by 20 to 24% in the 2- and 4-week treatments compared to the 6-week

treatment (p < 0.01).

Potassium, N, Cu, and pH showed significant interactions of solarization treatment

x duration (Table 4-1). Potassium was 73% higher in 4-week solarized treatments when

compared to the 4-week non-solarized treatments (p < 0.05). Among durations of

solarization, K was highest in the 4-week treatment and lowest in the 2-week treatment (p

< 0.05). Nitrogen was 19% higher in the 2-week solarized treatment compared to the 2-

week non-solarized treatment, and about 16% lower in the 2-week non-solarized

compared to the 4- and the 6-week non-solarized treatments (p < 0.05). Copper was

significantly lower in each duration of solarized treatment compared to non-solarized

treatments, and Cu was highest in the 6-week non-solarized treatment than in the 2-week

non-solarized treatment (p < 0.05). We found slightly lower pH in solarized plots of 2-

60

week and 6-week durations compared to non-solarized plots (Table 4-2). Soil pH was

highest in the 6-week non-solarized treatment compared to the 2- and 4-week non-

solarized treatments (p < 0.05). With the exception of K (Table 4-1), none of the mineral

nutrients or soil properties was affected by the duration of solarization treatment.

In 2004, the concentration of Mn in solarized treatments was 22% higher than non-

solarized treatments (p < 0.05; Table 4-3). Soil K was 54% higher in solarized plots (p <

0.01) and the concentration increased as duration increased (p < 0.1). Soil pH was

reduced (p < 0.01) in solarized treatments (Table 4-4). Organic matter (g kg-1

) was

significantly higher in solarized treatments compared to non-solarized treatments (p <

0.05).

Significant duration x solarization interactions were detected in the concentrations

of soil Na and Cu in 2004 (Table 4-3). Sodium was higher in 4- and 6-week solarization

treatments compared to the 2-week solarized treatment (p < 0.05). Sodium was also lower

in the 2-week solarized treatment compared to the 2-week non-solarized treatment (p <

0.01) but higher in the 6-week solarized treatment compared to the 6-week non-solarized

treatment (p < 0.1). Copper was 23% lower in the 6-week solarized treatment compared

to the 6-week non-solarized treatment. Among solarized duration treatments, Cu

concentration was higher in the 4-week treatment than in either the 2- or the 6-week

treatments (p < 0.05).

Okra Leaf Tissue Nutrients

During the 2003 experiment, the concentration of K in leaf tissue was 40% higher

in solarized treatments compared to non-solarized treatments (p < 0.01; Table 4-5).

Nitrogen concentration was also higher in solarized treatments than in non-solarized (p <

0.05) and concentration decreased as duration of treatment increased (p < 0.01). The

61

concentration of Mn was 29% higher in solarized treatments than non-solarized

treatments (p < 0.05).

In 2003, ANOVA of the concentrations of Mg, P, Na, and Zn in okra leaf tissue

resulted in significant interactions between solarization duration and treatment (Table 4-

5). Magnesium was 30% lower in 6-week solarized treatments compared to 6-week non-

solarized treatments (p < 0.05). Phosphorous concentration was 29% lower in the 6-week

solarized treatment compared to the 6-week non-solarized treatment, and among non-

solarized durations, the 6-week treatment was higher than the 2- and 4-week treatments

(p < 0.05). Among solarized treatments, P was higher in the 4-week treatment than in the

6-week treatment (p < 0.05). Sodium was 40% higher in the 4-week solarized treatment

compared to the 4-week non-solarized treatment (p < 0.05). The concentration of Zn was

29% lower in the 6-week solarized treatment than in the 6-week non-solarized treatment

(p < 0.05).

In 2004, the K concentration in okra leaf tissue was higher (p < 0.05) in solarized

treatments (Table 4-6). Potassium concentration in leaves was higher in 6-week

treatments than in the 4-week treatments (p < 0.05). No other leaf tissue nutrient

concentration showed any treatment effects in 2004.

Okra Biomass

In 2003, the dry whole plant biomass of the okra crop was more than three times

higher in the 4-week solarized treatment and four times higher in the 6-week solarized

treatment compared to the non-solarized treatments of the same duration (p < 0.01; Table

4-7). Okra biomass was 67% lower in the 6-week non-solarized compared to the 2-week

non-solarized treatment (p < 0.05). In 2004, okra biomass was almost three times higher

in the 4-week solarized treatment than in the 4-week non-solarized treatment (p < 0.01).

62

There were no other significant differences in biomass based on solarization treatment or

duration of treatment in 2004.

Discussion

Several soil mineral nutrients were affected by solarization treatments. Extractable

N increased only in 2-week solarized soil in 2003, but not in 2004. The inconsistency

between experimental years may have been due to the application of herbicide in the

control plots in 2004 or the sandy soil type, which had relatively low levels of organic

matter (1%). In both years, K and Mn in soil were higher in solarized treatments. This

effect may have been due to reduced leaching by rainfall under solarization plastic during

treatment; the amount of rainfall during solarization treatment was 20.9 cm in 2003 and

36.9 cm in 2004, with some 2-3 day periods receiving up to 7.6 cm (Florida Automated

Weather Network, 2005). We also saw a decrease in Cu with solarization treatment in

both years, a finding that is consistent with earlier research (Stapleton et al., 1985). In

2003, Zn decreased with solarization as well. In 2004, Na increased in 4- and 6-week

solarization treatments, which is consistent with a previous 6 to 7 week solarization

treatment on sandy soil (Chen and Katan, 1980). Soil pH decreased significantly with

solarization treatment, although the maximum difference was 0.3, in the 2- and 6-week

durations in 2003 and overall in 2004. Although these pH differences were small, this

deviation from previous research, which found no change or inconsistent changes in pH

in solarized soils (Chen and Katan, 1980; Stapleton et al. 1985), may be due to the sandy

soil type, sampling methods, or analytical methods.

Prior solarization research examining plant tissue found no significant differences

in nutrient concentrations (Stapleton et al., 1985). However, in the current experiment, K

concentration in leaf tissue increased with solarization treatment in both years. This

63

increase may reflect the increase in K in solarized soil, a decrease in K in non-solarized

plots due to weed competition, or may be influenced by other chemical and physical

changes in the soil. In 2003, N, Mn, and Na increased in leaf tissue in solarized

treatments, while Mg, P, and Zn decreased. The increase in N and Mn in leaf tissue may

reflect the increase of these nutrients in solarized soil and may also be considered an

indicator of overall plant health. In 2004, there were no differences in the concentration

of nutrients other than K. The lack of differences in leaf tissue nutrient concentration

2004 may be attributed to the decrease in plant replication and the plant damage caused

by the hurricanes.

In 2003, okra biomass tripled in the 4-week solarized treatment and quadrupled in

the 6-week solarization treatment compared to the non-solarized treatments of the same

duration. The increase in okra biomass in 4- and 6-week solarized treatments is in part

due to a decrease in weed competition by solarization (Seman-Varner, Chapter 3) and a

likely reflection of overall plant health. The data from 2004 indicate a significant increase

in okra biomass in 4-week solarized treatments compared to 4-week non-solarized

treatments, while no significant difference appears between the 6-week treatments. This

deviation from the data for the first year is due to the application of herbicide to the non-

solarized plots, which would have impacted the 6-week non-solarized plots most because

these had the most mature weeds at the time of herbicide application. Lower leaf tissue

concentrations of Mn in 2004 compared to 2003 may have impacted okra biomass. The

data from 2003 suggest that solarization durations of 4- and 6-weeks are equally effective

and significantly better in increasing crop biomass than the 2-week solarization treatment.

64

Conclusion

In general, the changes in soil mineral nutrients were reflected in changes in leaf

tissue nutrients, with only a few exceptions. Data from 2003 were more reflective of the

effects of solarization on soil chemistry, leaf nutrients, and plant biomass, due to the

herbicide application and anomalous weather in 2004. Overall, solarization increased

concentrations of essential nutrients. This increase in nutrients was reflected in the leaf

tissue analysis and increased biomass that indicated an improvement in crop health due to

solarization. The increase in okra biomass in solarization treatments of 4- and 6-week

durations indicates that okra plants utilized the increased nutrients available and that

solarization did not limit the availability of an organic nutrient source. This study also

indicates an increased growth response that may involve changing soil chemical and

physical properties, which adds to the benefits of using solarization for soil-borne pest

management.

Table 4-1. Extractable soil mineral nutrient concentrations from 2003 summer solarization experiment (0 days post-treatment).

Soil nutrient concentrations (mg kg-1

)

____________________________________________________________________________________________________________

Durx Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean Sol Non Mean

-------------N-------------- -------------P------------ -----------K------------- ------------Ca------------ ------------Mg----------

2 406 Ay 340 B*

z 373 38.9 38.1 38.5 21.4 C 22.7 A 22.1 103.4 115.0 109.2 10.8 13.0 11.9

4 385 A 408 A 397 38.6 46.5 42.6 37.0 A 21.4 A* 24.2 97.5 93.9 95.7 11.8 9.0 10.4

6 381 A 406 A 394 40.3 47.6 44.0 30.6 B 26.9 A 28.8 124.7 130.1 127.4 14.5 15.8 15.2

Mean 391 385 39.3 44.1 26.4 23.7 108.6 113.0 12.4 12.6

------------Zn------------- ------------Cu----------- -----------Mn------------ -------------Fe------------- -----------Na----------

2 1.2 1.4 1.3 0.25 A 0.27 B* 2.1 2.0 2.0 9.9 10.7 10.3 7.4 7.3 7.4 B

4 1.4 1.4 1.4 0.26 A 0.29 AB* 2.2 2.0 2.1 10.5 11.5 11.0 8.5 7.1 7.8 B

6 1.3 1.5 1.4 0.24 A 0.31 A* 2.4 1.7 2.1 9.8 11.4 10.6 10.1 9.4 9.8 A

Mean 1.3 1.5** 0.25 0.29 2.2 1.9* 10.1 11.2 8.7 8.0 65

x Duration (Dur) of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 12 August 2003.

y Means in columns followed by the same letter do not differ at p < 0.05 according to LSD test. No letters in column indicates no

significant differences. z * and ** indicate significant differences between solarized and non-solarized at p < 0.05 and 0.01, respectively. No symbol indicates

no significant difference.

Table 4-2. Soil properties from 2003 summer solarization experiment (0 days post-treatment).

pH Organic matter (g kg-1

) CEC (cmol kg-1

)

___________________ ________________________ _____________________

Durx Sol Non Mean Sol Non Mean Sol Non Mean

2 5.2 Ay 5.3 B*

z 5.3 10.5 11.0 10.7 2.7 2.5 2.6

4 5.3 A 5.3 B 5.3 10.6 12.2 11.4 2.6 2.6 2.6

6 5.3 A 5.6 A* 5.4 11.5 12.0 11.8 2.9 2.7 2.8

Mean 5.3 5.4 10.9 11.7+ 2.7 2.6* x Duration (Dur) of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 12 August 2003.

y Means in columns followed by the same letter do not differ at p < 0.05 according to LSD test. No letter indicates no significant

difference. z +, and * indicates significant differences between solarized and non-solarized at p < 0.10 and 0.05, respectively. No symbol indicates

no significant difference.

66

Table 4-3. Extractable soil mineral nutrient concentrations from 2004 summer solarization experiment (4 days post-treatment).

Soil nutrient concentrations (mg kg-1

)

____________________________________________________________________________________________________________


-------------N----------- -----------P------------ -----------K------------- ------------Ca-------------- ------------Mg----------

2 357 338 347 73.0 74.4 73.7 27.2 21.6 24.4 B 158.2 148.2 153.2 22.6 20.6 21.6

4 352 364 358 90.8 81.1 86.0 44.7 25.0 34.9 AB 260.1 200.0 230.0 39.3 29.3 34.3

6 355 347 351 79.1 84.0 81.6 47.8 31.2 39.5 A 187.4 195.0 191.2 33.5 34.2 33.8

Mean 355 350 81.0 79.8 39.9 25.9***z 201.9 181.1 31.8 28.0

------------Zn--------- ------------Cu-------------- ---------Mn---------- -------------Fe----------- ----------Na----------

2 1.4 1.7 1.6 0.26 By 0.26 A 0.26 3.3 2.6 2.9 18.1 18.8 18.4 3.6 B 4.5 A** 4.0

4 2.1 1.9 2.0 0.32 A 0.28 A 0.28 3.6 2.8 3.2 19.1 18.6 18.8 5.3 A 5.0 A 5.2

6 1.8 1.8 1.8 0.23 B 0.30 A* 0.26 3.1 2.8 2.9 17.6 18.7 18.2 5.8 A 4.6 A+ 5.2

Mean 1.8 1.8 0.27 0.27 3.3 2.7** 18.3 18.7 8.7 8.0 67



difference. z +, *, **, and *** indicate significant differences between solarized and non-solarized at p < 0.10, 0.05, 0.01, and 0.001, respectively.

No symbol indicates no significant difference.

Table 4-4. Soil properties from 2004 summer solarization experiment (4 days post-treatment).

pH Organic matter (g kg-1

) CEC (cmol kg-1

)

___________________ ____________________ ____________________

Dury Sol Non Mean Sol Non Mean Sol Non Mean

2 5.6 5.8 5.7 11.1 11.1 11.1 3.61 3.44 3.52

4 5.8 6.0 5.9 10.8 11.2 11.0 4.05 3.65 3.85

6 5.8 6.0 5.9 11.2 10.7 11.0 3.72 3.74 3.73

Mean 5.7 5.9**z 11.1 11.0* 3.79 3.61

y Duration (Dur) of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 12 August 2003.

z *, and ** indicates significant differences between solarized and non-solarized at p < 0.05 and 0.01, respectively. No symbol


68

Table 4-5. Okra leaf tissue nutrient concentrations at conclusion of 2003 summer solarization experiment (59 days post-treatement).

Okra leaf nutrient concentrations

____________________________________________________________________________________________________________


--------N (g kg-1

)-------- -------P (g kg-1

)------- --------K (g kg-1

)-------- -------Ca (g kg-1

)------- --------Mg (g kg-1

)--------

2 41.2 38.6 39.9 Ay 4.3 AB 4.4 B 4.3 33.4 28.2 30.8 66.4 55.4 60.9 7.0 A 8.4 A 7.7

4 37.3 29.2 33.3 B 4.9 A 4.9 B 4.9 38.1 22.2 30.2 56.2 67.3 61.8 7.6 A 7.3 A 7.4

6 30.1 29.2 29.7 B 4.0 B 5.6 A* 4.8 33.4 24.3 28.9 55.6 60.6 58.1 6.1 A 8.7 A* 7.4

Mean 36.2 32.3*z 4.4 5.0 35.0 24.9*** 59.4 61.1 6.9 8.1

-------Zn (mg kg-1

)------ ------Cu (mg kg-1

)------- ------Mn (mg kg-1

)----- --------Fe (mg kg-1

)----- --------Na (g kg-1

)--------

2 102 A 102 A 102 7.2 7.4 7.3 442 327 385 196 188 192 0.62 A 0.55 A 0.58

4 123 A 129 A 126 9.0 7.6 8.3 515 440 478 298 156 227 0.76 A 0.54 A* 0.65

6 99 A 139 A* 119 6.0 10.8 8.4 518 373 445 154 168 161 0.70 A 0.75 A 0.72

Mean 108 123 7.4 8.6 492 380* 216 171 0.69 0.61 69



difference. z *, and *** indicate significant differences between solarized and non-solarized at p < 0.05 and 0.001, respectively. No symbol


Table 4-6. Okra leaf tissue nutrient concentrations at conclusion of 2004 summer solarization experiment (65 days post-treatment).

Okra leaf nutrient concentrations

____________________________________________________________________________________________________________


-------N (g kg-1

)-------- --------P (g kg-1

)----- ---------K (g kg-1

)---------- ------Ca (g kg-1

)------- -----Mg (g kg-1

)------

2 49.9 51.7 50.8 4.3 4.5 4.5 27.6 23.9 25.7 ABy 25.9 26.7 26.3 6.9 8.5 7.7

4 48.3 49.6 49.0 4.2 4.3 4.3 24.9 24.8 24.8 B 25.7 26.6 26.2 7.3 7.6 7.4

6 50.6 50.0 50.3 4.5 4.5 4.5 30.3 24.3 27.3 A 26.2 24.2 25.2 7.2 7.2 7.2

Mean 49.6 50.4 4.3 4.4 27.6 24.3*z 25.9 25.9 7.1 7.8

----Zn (mg kg-1

)----- ------Cu (mg kg-1

)----- -----Mn (mg kg-1

)------ --------Fe (mg kg-1

)----- ------Na (g kg-1

)-----

2 67 72 70 6.8 8.0 7.4 140 124 132 100 108 104 0.8 0.6 0.7

4 62 71 67 5.6 6.2 5.9 108 115 111 110 94 102 0.8 0.8 0.8

6 62 69 66 9.2 7.8 8.5 72 90 81 114 110 112 0.8 0.8 0.8

Mean 63 71 7.2 7.3 107 110 108 104 0.8 0.7 70



difference. z +, *, **, and *** indicate significant differences between solarized and non-solarized at p < 0.10, 0.05, 0.01, and 0.001, respectively.

No symbol indicates no significant difference.

Table 4-7. Dry okra biomass at conclusion of summer solarization experiments, 2003 (56 days post-treatment) and 2004 (67 days

post-treatment).

Dry weight of okra biomass (g)

________________________________________________________________________

2003 2004

_________________________ _________________________

Durationx Sol Non Mean Sol Non Mean

2 181.0 Ay 161.8 A

z 171.4 37.8 A 43.6 A 40.7

4 247.4 A 81.6 AB** 164.5 106.3 A 35.7 A** 71.0

6 221.1 A 54.1 B*** 137.6 96.8 A 85.2 A 91.0

Mean 216.5 99.2 80.3 54.9 x Duration of solarized (Sol) and non-solarized (Non) treatments in weeks, ending on 12 August 2003 and 11 August 2004.

y Means in columns followed by the same letter do not differ at p < 0.05 according to LSD test; no letters in column indicate no

differences at p < 0.05. 71

z ** and *** indicate significant differences between solarized and non-solarized at p < 0.01 and 0.001, respectively.

CHAPTER 5

EFFECTS OF SOLARIZATION ON RELATIONSHIPS BETWEEN OKRA

BIOMASS, LEAF NUTRIENT CONTENT, AND SOIL PROPERTIES

Introduction

Recent restrictions on soil fumigants including methyl bromide have caused

increasing research emphasis on non-chemical methods of soil disinfestation. Modern

soil solarization developed in the 1970s and expanded to 38 countries by 1990 (Katan and

DeVay, 1991). Within the last quarter century, soil solarization has been one of the most

researched non-chemical methods of soil-borne pest management. This technique has

successfully been used in the management of fungal, bacterial, nematode, insect, and

weed pests.

Soil solarization involves the application of thin (25-30µm), clear polyethylene or

polyvinyl chloride plastic that transmits solar radiation to heat the underlying soil and

subsequently kills soil-borne pests (Katan, 1981; Katan and DeVay, 1991; McGovern and

McSorley, 1997). During solarization treatments, the upper 30 cm of the soil is heated to

temperatures usually within the range of 30-60°C, depending on soil type, soil moisture,

climate, and treatment enhancements (Katan, 1981). Over the course of treatment,

generally 4 to 8 weeks, the soil is diurnally heated with temperatures highest at shallower

depths and maximum temperatures reached in the afternoon hours. The cyclic heating of

the soil results in lethal temperatures sustained long enough to kill a variety of pests.

Once the plastic is removed, the soil is disinfested and ready for planting.

72

73

Many studies have concluded that solarization positively affects crop yield by

managing soil-borne pests (see reviews by Davis, 1991; Elmore, 1991; McGovern and

McSorley, 1997). While the effects of solarization on soil organisms and crop yield have

been extensively studied, its influence on extractable soil mineral nutrients has not been

thoroughly researched. One review by Chen et al. (1991) cited examples of research

conducted on the effects of sterilization, steam treatment, fumigation, and solarization on

extractable soil mineral content. Solarized soils contained higher levels of NHB4B

+, NO B3B

-,

N, Ca2+

, and Mg2+

(Chen and Katan, 1980). Phosphorous, K and Cl also increased in

some soils (Chen et al., 1991). The few studies on soil mineral effects have not examined

the effect solarization may have on relationships between soil nutrients and plant mineral

nutrients. This important information will add to the understanding of crop nutrition in

solarized soils.

The objectives of this study were to examine the indirect effects of solarization on

the relationships between leaf biomass, soil nutrients, and leaf nutrient content. We

specifically focused on soil and plant mineral nutrients including N, P, K, Ca, and Mg.

We examined how solarization may have indirectly changed the relationships between

leaf biomass and soil mineral nutrients. Then we examined the indirect effect of

solarization on the relationships between nutrients found in leaf tissue and those in the

soil.

Methods

This experiment was conducted at the University of Florida’s Plant Science

Research and Education Unit (PSREU) near Citra, Florida, during the summer of 2003.

The soil at the study site was a hyperthermic, uncoated Candler Series sand with a 0 to

5% slope (Thomas et al., 1979). The soil texture was 95% sand, with only 2% silt and 3%

74

clay. Soil pH ranged from 5.5 to 6.0 with an average of 5.9. The field was prepared with

a crimson clover (Trifolium incarnatum L. ‘Dixie’) cover crop during the winter season

and disked two days before beds were constructed.

We used a split-plot experimental design where the main effect was duration of

treatment and the sub-effect was solarization. Five replicates were arranged in a

randomized complete block design on the main effect. Each experimental plot was a

raised bed 6 m long, 1 m wide, and 20 cm high. The soil was moistened by overhead

irrigation if it was not sufficiently moist before plastic application. The solarization

treatment was installed using a single layer of clear, 25-µm-thick, UV-stabilized, low-

density polyethylene mulch (ISO Poly Films, Inc., Gray Court, SC). Solarization

treatments and non-solarization control treatments of 2, 4, and 6 week durations were

conducted during July and August. The treatments began on sequential dates and

concluded on 12 August.

Six soil cores 2.5 cm in diameter and 15 cm deep were collected from each plot

immediately following treatment. The cores were composited and a subsample was

removed for the analysis of Mehlich I extractable (Mehlich, 1953) nutrients (N, P, K, Ca,

Mg, Na, Cu, Fe, Mn, Zn) and properties (pH, CEC, OM; Marshall et al., 2002). Solutions

were made by standard procedures. Nitrogen was analyzed with an aluminum block

digester (Gallaher et al., 1975) and analyzed by a modified micro Kjeldahl procedure

(Marshall et al., 2002). Phosphorous was analyzed with colorimetry. Flame emission

spectrophotometry was used for K concentration. Calcium, Mg, Na, Cu, Fe, Mn and Zn

concentrations were analyzed using atomic absorption spectrophotometry (Gallaher et al.,

1973).

75

Okra (Hibiscus esculentus L. ‘Clemson spineless’) seed was planted and

germinated in a growth room, then moved to a greenhouse for maturation. The seedlings

were watered and fertilized with a 20-20-20 (N-P B2BO B5B-K B2BO) mix as needed for 5 weeks.

One week after the conclusion of solarization treatments, okra seedlings were planted in

the experimental plots. An organic fertilizer of green cowpea (Vigna unguiculata (L.)

Walp. ‘Iron Clay’) hay was applied on the soil surface to the area immediately around the

okra seedlings at a rate of 3.5 kg m-2

. The cowpea was analyzed for nutrient

concentration at the time of application. The okra was harvested 6 weeks after planting, at

early flowering. Leaf tissue was collected, dried, weighed and ground for nutrient

analysis. For the analysis of both okra and cowpea plant material, solutions were

prepared and nutrient content was analyzed using standard procedures similar to those

used for soil mineral analysis (Gallaher et al., 1973; Gallaher et al., 1975).

Okra leaf nutrient concentrations (g kg-1

) were transformed into nutrient contents

(mg of nutrient/leaf) by multiplying the concentration by the total dry matter per number

of leaves in the sample. Leaf dry matter, leaf nutrient content, and extractable soil

nutrient data were analyzed by computing correlation coefficients (r) in solarized and

non-solarized treatments using MSTAT-C software (Michigan State University, East

Lansing, MI).

Results

Leaf tissue analysis of the cowpea green hay provided the nutrient concentration of

the organic fertilizer, which included 21.0 g kg-1

N (Table 5-1). Okra leaf dry matter was

positively correlated with soil Ca (r = 0.64; p < 0.01) and Mg (r = 0.61; p < 0.05) in

solarized treatments but not correlated (r = 0.43 for Ca, r = 0.46 for Mg; p > 0.05) in non-

solarized plots. The correlation coefficients of dry matter with other nutrients, including

76

N, P, and K, were not significant (p > 0.05) in solarized or non-solarized treatments (data

not shown). Correlations of the same mineral nutrient in the leaf tissue with its

concentration in soil were positive and highly significant (p < 0.02) for Ca and Mg and

significant (p < 0.10) for N, Mn, and Zn in solarized plots (Table 5-2). When correlating

different plant nutrients with soil mineral nutrients, we found several positive

relationships (p < 0.05) in solarized plots, while the non-solarized plots did not show

significant (p > 0.05) relationships (Table 5-3).

Discussion

Leaf biomass was positively correlated with leaf Ca content and leaf Mg content in

solarized plots. Calcium is an important nutrient involved in the structural support of the

plant cell wall. Magnesium is essential to photosynthesis and energy production (Mills

and Jones, 1991). Both nutrients positively influenced leaf production and biomass. The

lack of significant correlation between leaf biomass and leaf nutrient content in non-

solarized treatments may be due to weed competition.

Essential nutrients available in the soil should show a positive correlation with the

same nutrients in leaf tissue. This expectation was supported by the data in the solarized

plots, with significant positive correlation coefficients for Ca, Mg, N, Mn, and Zn in soil

and leaf tissue (p < 0.1). Calcium in soil and Ca in the leaf, and Mg in soil and Mg in the

leaf were more strongly correlated than the other nutrients with r = 0.75 and r = 0.58,

respectively. These correlations may be due to the concentration of K in the soil, which

may have limited Ca and Mg uptake by the plant (Gallaher and Jellum, 1976). As the N,

Mn, and Zn content in soil increased, the content in the plant also increased. This

relationship can be explained by the necessity of N or by the limited availability of Mn

and Zn.

77

The correlation coefficients relating mineral nutrients in the plant to different

mineral nutrients in the soil in solarized plots show positive relationships. Both Ca and

Mg positively influence P uptake, as these data support. It is thought that Ca stimulates

the transport of P, and Mg activates enzymes involving P (Mills and Jones, 1991). A

positive correlation was also expected between K and N due to their close functioning,

and this was indeed the case. Nitrogen measured was in elemental form and so we cannot

deduce what mechanism of influence it may have had on Ca and Mg, although the data

suggest a positive correlation. An inverse relationship was expected between Ca and Mg,

but we found a positive correlation. This deviation may be due to concentration levels,

other soil chemical properties (such as soil K), or the specific needs of okra plants.

In solarized plots, the strength of relationships between leaf biomass and

extractable soil nutrients and between leaf nutrient content and extractable soil nutrients

is probably due in part to a decrease in weed competition directly caused by solarization.

The high weed population in non-solarized plots would have influenced leaf biomass and

nutrient concentration through direct competition and indirectly altered the relationships

between leaf biomass, leaf nutrient content, and levels of extractable soil nutrients.

Conclusion

Solarization improved plant growth and nutrient uptake as is represented in the

positive correlations of leaf biomass, leaf nutrient content, and soil nutrients in the

solarized plots. The lack of significant correlation coefficients in the non-solarized plots

suggests some interference, possibly weed competition, reduced soil nutrient

mineralization due to lower soil temperatures, or increased leaching due to exposure to

rainfall. Solarization benefited the plant and positively influenced nutrient uptake from

78

the soil and from an organic fertilizer source. The correlations suggest that solarization

facilitates growth and nutrient uptake by the plant.

79

Table 5-1. Cowpea green fertilizer nutrient concentration and content for each 3.5 kg m-2

application.

________________________________________________________________

Cowpea nutrients Concentration Fertilizer nutrient content

________________________________________________________________

----------g kg-1

----------- ---------- mg m-2

----------

N 21.0 18.4

P 3.3 2.9

K 28.4 33.6

Ca 13.9 12.2

Mg 3.1 2.7

________________________________________________________________

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Table 5-2. Correlation coefficients (r) and levels of significance (p) of the relationship

between okra leaf tissue nutrient content and extractable soil nutrients in

solarized treatments.

___________________________________

Plant/Soil+ r p

___________________________________

Ca/Ca 0.75 <0.01

Mg/Mg 0.58 0.02

N/N 0.49 0.06

Mn/Mn 0.45 0.09

Zn/Zn 0.50 0.06

___________________________________

+ indicates nutrient content of okra leaf tissue correlated with extractable soil nutrient

81

Table 5-3. Correlation coefficients (r) and level of significance (p) for correlations

between leaf tissue nutrients and soil minerals in solarized and non-solarized

treatments.

___________________________________________________________

Solarized Non-solarized

_________________ ________________

Plant/Soil+

r p r p

____________________________________________________________

Ca/Mg 0.68 <0.01 0.38 0.16

Mg/Ca 0.59 0.02 0.49 0.06

Mg/N 0.57 0.03 -0.19 0.49

K/N 0.51 0.05 -0.20 0.48

P/Ca 0.58 0.02 0.43 0.10

P/Mg 0.57 0.03 0.48 0.07

N/Ca 0.61 0.02 0.40 0.13

N/Mg 0.57 0.03 0.43 0.11

____________________________________________________________

+ indicates nutrient content of okra leaf tissue correlated with extractable soil nutrient

CHAPTER 6

CONCLUSIONS

Solarization experiments conducted during summer 2003 and summer 2004 had

various results in the management of specific organisms. Although the experimental site

did not have an infestation of nematodes initially, nematodes recolonized solarized plots

at a slower rate than non-solarized plots. The decrease in recolonization rate in solarized

treatments was likely aided by the significant reduction in alternative weed hosts in

solarized plots compared to non-solarized plots. Duration of solarization did not affect

nematode population levels. Collembola and mites were generally reduced by

solarization and remained lower in solarized plots than in non-solarized plots up to 2

months post-treatment. Duration of solarization treatment did not affect microarthropod

populations. Despite the reduction in microarthropods by solarization, the availability of

nutrients from an organic fertilizer was not affected, suggesting the importance and rapid

recovery of fungal and bacterial detritivores.

Solarization significantly reduced weed cover throughout the experiment (up to 65

days post-treatment), although there were no differences among duration (2, 4, and 6

weeks) of solarization. The number of plants or stems was greatly reduced by solarization

as well. In 2003, non-solarized plots reached 100% coverage by 28 days post-treatment.

Individual species were affected differently in 2003. Indigofera hirsuta was more

effectively reduced by 4- and 6-week solarization than by the 2-week treatment. Cyperus

spp. were reduced initially by the 4- and 6-week solarization treatments, but differences

were not detected post-treatment when numbers remained low in solarized plots and were

82

83

reduced in non-solarized plots by competition by other weed species. Weed biomass was

reduced by 90% due to solarization in 2003. The data from 2004 were confounded by the

application of herbicide to some of the control plots, making some analysis unclear and

changing the species composition of the weed plots.

Potassium and Mn consistently increased in solarized soil, while Cu decreased.

These data do not support earlier findings of increased NHB4B

+ and NO B3B

- concentration in

soil (Chen and Katan, 1980; Stapleton et al., 1985), which may be due to soil differences,

anomalous weather, or differences in sampling and analysis. Another unique result of this

experiment is a slight reduction in soil pH due to solarization; this too may be related to

soil type, sampling, or analysis. Concentrations of K in leaf tissue consistently increased

in solarized treatments, another unique result that may be a reflection of K increase in the

soil. In 2003, concentrations of leaf tissue N, Mn and Na also increased, while Mg, P, and

Zn decreased. These results were not repeated in 2004, possibly due to experimental

differences caused by weather or herbicide.

In 2003, okra biomass was tripled in 4-week solarization treatments and quadrupled

in 6-week solarization treatments compared to non-solarized treatments of the same

durations. There was no difference between 2-week solarized and non-solarized

treatments. The increased biomass is likely due to a reduction in weed competition in the

4- and 6-week solarized plots. Based on these data on okra performance, the availability

of an organic nutrient source was not hindered by solarization. These data also suggest

that solarization treatments of 4- and 6-weeks are more effective in increasing overall

crop health and biomass than a 2-week treatment, not only due to pest reduction but also

84

due to an increased growth response associated with improved soil nutrients and

properties.

Examination of the relationships between soil nutrient content and properties, leaf

tissue nutrient content, and leaf biomass, also supported the result that solarization

improved overall plant health. Biomass was positively correlated with Ca and Mg

contents in the leaf in solarized treatments, reflecting the importance of these nutrients to

leaf structure and photosynthesis. In solarized treatments, there were also positive

correlations between Ca, Mg, N, Mn and Zn in soil and the same nutrient in leaf tissue. In

general, nutrients in soil were positively correlated with nutrients in leaf tissue in

solarized treatments, while the same correlations were insignificant or weaker in non-

solarized treatments. These differences between solarized and non-solarized treatments

are likely due to weed competition in non-solarized plots that would have interfered with

the relationships between soil nutrients and properties, leaf tissue nutrients, and leaf

biomass.

Solarization was an effective method of managing weeds, nematodes,

microarthropods, and soil and crop nutrients. Solarization reduced nematode, Collembola

and mite populations; although solarization did not hinder the availability of nutrients

from an organic fertilizer source. Four and 6-week solarization durations were more

effective in reducing populations of certain weed species (e.g. Indigofera hirsuta and

Cyperus spp.). Solarization increased K and Mn in soil and subsequently K in leaf tissue.

Solarization treatments of 4- and 6-week durations were more effective in increasing crop

biomass than the 2-week treatment. In general, these data suggest that the optimal

solarization duration for summer in north Florida is 4- or 6-weeks, but may vary for

85

management foci or target species. These data also suggest that solarization is a useful

management technique due to effective pest management (weeds and nematodes), and

increased growth responses related to soil chemistry and properties. This non-chemical

method of soil disinfestation could be further optimized by more studies on individual

organisms, particularly weed species, and including research on soil fungi and bacteria.

Solarization continues to be an effective, versatile, and environmentally-sound alternative

to the use of soil fumigants.

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BIOGRAPHICAL SKETCH

Rachel Seman-Varner first became interested in natural science as a young girl. She

spent many afternoons exploring the small patches of forest that remained around her

suburban Tampa home. During her senior year of high school, her academic interests

shifted from literature to science because of her first physics class and the incredible

teacher who inspired her there. As a freshman she enrolled in an introductory botany

class and decided then on her major. During her last year in the Department of Botany at

the University of Florida, Rachel was honored with an undergraduate research

scholarship. She developed hypotheses and methods, conducted research, and analyzed

and presented an independent research project on the eco-physiology of south Florida

mangroves. After graduation, Rachel delayed her application to graduate school and

accepted an internship on an organic farm in Gainesville. On the farm she learned how

scientific research, plant ecology, and environmental conservation could be merged to

improve sustainable agricultural practices. Her entry into a master’s program, with a

focus on agricultural ecology, marked the synthesis of love of the natural world,

education in plant biology, experience in scientific research, and dedication to

sustainability. It was during her master’s program that her loving and inspiring marriage,

and the birth of her first daughter reinforced the necessity to secure a sustainable future.

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